However, even if you have a focus to take the place of the material components, in this case casting with the components may be strongly preferred so that you can obtain the full benefit of the optional protective circle: "As part of casting the spell, you can form a circle on the ground with the blood used as a material component." The spell may be telling us that four ounces of blood is the minimum needed to make a circle around "your space", such that with less blood you might be able to complete the spell, but not the circle. Similarly, a vial is the ideal container to efficiently trace the circle with blood - were the blood held in a container like a bowl, you might need much more to complete the circle.

However, even if you have a focus to take the place of the material components, in this case casting with the components may be strongly preferred so that you can obtain the full benefit of the optional protective circle: "As part of casting the spell, you can form a circle on the ground with the blood used as a material component." The spell may be telling us that four ounces of blood is the minimum needed to make a circle around "your space", such that with less blood you might be able to complete the spell, but not the circle. Similarly, a narrow-mouthed vial, with which one can carefully control the pour rate of the blood, is the ideal container to efficiently trace the circle - were the blood held in a container like a bowl, you might need much more to complete the circle without leaving any gaps.

A vial is (probably) not needed

We should remember also the rule that material components may be completely substituted by an component pouch or an arcane focus so long as the components don't have a specified cost. Thus for Summon Elemental the vial itself really is an integral, noninterchangeable part of the spell, whereas for Summon Greater Demon both the vial and the blood can be substituted by other things. It is reasonable to assume that a component pouch contains spare vials, less so that it has a constant supply of fresh humanoid blood, while a wooden wand has neither. However, as Adeptus points out, the actual blood is necessary to draw the protective circle, which is an optional part of the casting.

A vial is likely not required, but enough blood is a good idea

However, even if you have a focus to take the place of the material components, in this case casting with the components (or at least the blood in any container) may be strongly preferred so that you can obtain the benefit of the optional protective circle. The spell may be telling us that four ounces of blood is the minimum needed to make a circle around "your space", such that with less blood you might be able to complete the spell, but not the circle.

A vial is (probably) not needed but is a good idea

We should remember also the rule that material components may be completely substituted by an component pouch or an arcane focus so long as the components don't have a specified cost. Thus for Summon Elemental the vial itself really is an integral, noninterchangeable part of the spell, whereas for Summon Greater Demon both the vial and the blood can be substituted by other things. It is reasonable to assume that a component pouch contains spare vials, less so that it has a constant supply of fresh humanoid blood, while a wooden wand has neither. However, as Adeptus points out, the actual blood is necessary to draw the protective circle, which is an optional part of the casting.

A vial is likely not required, but preferred

However, even if you have a focus to take the place of the material components, in this case casting with the components may be strongly preferred so that you can obtain the full benefit of the optional protective circle: "As part of casting the spell, you can form a circle on the ground with the blood used as a material component." The spell may be telling us that four ounces of blood is the minimum needed to make a circle around "your space", such that with less blood you might be able to complete the spell, but not the circle. Similarly, a vial is the ideal container to efficiently trace the circle with blood - were the blood held in a container like a bowl, you might need much more to complete the circle.

It's a fair question

On the one hand, "a vial of blood" could mean you need a literal vial. On the other hand, perhaps you need a 'vial's worth of blood', since a vial is a unit of measurement. A vial is, in fact, on the PHB list of Container Capacity as officially holding four ounces of liquid. So it is a fair question as to whether you need 'blood inside a vial', or whether 'exactly four ounces of blood in any sort of container' will do. Given the ambiguity of meaning in 'a vial of blood', we won't be able to settle this by looking at the spell description itself, but we can look at other spells and see whether any patterns emerge.

Do spells ever specify exact measures?

A lot of spells have sympathetic magic material components in which a small amount of a symbolic substance is used. For the dozens of spells that measure their components in "pinches", "bits", "pieces" (or "small pieces"), "handfuls", and "drops", I was able to find only one that unambiguously calls for a specific measured amount. Clone requires "at least one cubic inch of flesh". It would thus be very unusual for Summon Greater Demon to require exactly four ounces of blood. However, there are a number of other spells that are similar to Summon Greater Demon in that they specify a liquid substance in a container that could be literal or could be a measurement. "A vial" is also used in Commune, Summon Lesser Demons, Rime's Binding Ice, and Create Magen. "A cup" is used in Armor of Agathys. We can't do anything but circular reasoning here; either they all indicate a specific measurement, or they all denote a specific container type, but without knowing which we can't infer anything about the spell in question.

Do spells ever specifically identify their components' containers?

We should remember also the rule that material components may be completely substituted by an component pouch or an arcane focus so long as the components don't have a specified cost. Thus for Summon Elemental the vial itself really is an integral, noninterchangeable part of the spell, whereas for Summon Greater Demon both the vial and the blood can be substituted by other things. It is reasonable to assume that a component pouch contains spare vials, less so that it has a constant supply of fresh humanoid blood, while a wooden wand has neither.

In conclusion

We know that if it was important to specify the container as part of the spell, this could be done, as it is in Hypnotic Pattern; by not only saying 'blood inside a vial' (rather than 'a vial of blood') but by telling us what material the vial would need to made from. We know that simply saying 'a vial of blood worth 1gp' would mean the component could not be substituted by a focus or pouch. We also know that if it was important to specify the exact amount of blood to be used, this could be done as well, as it is in the Clone spell. Since none of these details are specified, Summon Greater Demons appears to be like the vast majority of spells, whose material components are no more detailed than those of Insect Plague: "a few grains of sugar, some kernels of grain, and a smear of fat." In this spirit, it seems likely that all you need for the spell, absent a focus, is around four ounces of blood in some sort of container.

A vial is (probably) not needed

It's a fair question

On the one hand, "a vial of blood" could mean you need a literal vial. On the other hand, perhaps you need a 'vial's worth of blood', since a vial is a unit of measurement. A vial is, in fact, on the PHB list of Container Capacity as officially holding four ounces of liquid. So it is a fair question as to whether you need specifically 'blood inside a vial', or whether 'exactly four ounces of blood in any sort of container' will do. Given the ambiguity of meaning in 'a vial of blood', we won't be able to settle this by looking at the spell description itself, but we can look at other spells and see whether any patterns emerge.

Spells very rarely specify exact measures

A lot of spells have sympathetic magic material components in which a small amount of a symbolic substance is used. For the dozens of spells that measure their components in "pinches", "bits", "pieces" (or "small pieces"), "handfuls", and "drops", I was able to find only one that unambiguously calls for a specific measured amount. Clone requires "at least one cubic inch of flesh". It would thus be very unusual for Summon Greater Demon to require exactly four ounces of blood. However, there are a number of other spells that are similar to Summon Greater Demon in that they specify a liquid substance in a container that could be literal or could be a measurement. "A vial" is also used in Commune, Summon Lesser Demons, Rime's Binding Ice, and Create Magen (and "a cup" is used in Armor of Agathys). We can't do anything but circular reasoning here; either they all indicate a specific measurement, or they all denote a specific container type, but without knowing which we can't infer anything about the spell in question.

Spells sometimes call for specific liquid component containers

We should remember also the rule that material components may be completely substituted by an component pouch or an arcane focus so long as the components don't have a specified cost. Thus for Summon Elemental the vial itself really is an integral, noninterchangeable part of the spell, whereas for Summon Greater Demon both the vial and the blood can be substituted by other things. It is reasonable to assume that a component pouch contains spare vials, less so that it has a constant supply of fresh humanoid blood, while a wooden wand has neither. However, as Adeptus points out, the actual blood is necessary to draw the protective circle, which is an optional part of the casting.

A vial is likely not required, but enough blood is a good idea

We know that if it was important to specify the container as part of the spell, this could be done. As in Hypnotic Pattern, not only could the spell say 'blood inside a vial' (rather than 'a vial of blood'), but it could tell us what material the vial would need to made from. We know that simply saying 'a vial of blood worth 1gp' would mean the component could not be substituted by a focus or pouch. We also know that if it was important to specify the exact amount of blood to be used, this could be done as well, as it is in the Clone spell. Since none of these details are specified, Summon Greater Demons appears to be like the vast majority of spells, whose material components are no more detailed than those of Insect Plague: "a few grains of sugar, some kernels of grain, and a smear of fat." In this spirit, it seems likely that all you need for the spell, absent a focus, is around four ounces of blood in some sort of container.

However, even if you have a focus to take the place of the material components, in this case casting with the components (or at least the blood in any container) may be strongly preferred so that you can obtain the benefit of the optional protective circle. The spell may be telling us that four ounces of blood is the minimum needed to make a circle around "your space", such that with less blood you might be able to complete the spell, but not the circle.