From what I understand, the fact that the sun burns vampires comes from the religious significance of the sun, as intense light does not actually damage vampires beyond sending them into a frenzy.
So would a mage that is planning to create sunlight against a vampire need Spirit sphere?
According to the 64th page of the Choristers' Tradition Book, creation of sunlight is a Forces 2-3 plus Prime 2 Effect. I would think that if anyone is aware of the spiritual significance of sunlight, it should be the Singers, and yet the book does not suggest adding Spirit. However, the comment in the last paragraph of the Rote seems ambiguous to me:
Beings susceptible to sunlight might take one Health Level's worth of damage for every success over the fifth, at the Storyteller's option. This Effect does not duplicate the effects of sunlight, though it can be used to intensify existing sunshine (add one Health Level for every success).
The first quoted sentence seems to indicate that it's up to the ST to decide whether this damages sunlight-susceptible characters. However, the last sentence can be read either as saying that this is still not full-fledged sunlight in terms of effect or that it's not meant to allow doubling the effects of sunshine, but only to add to them. I'm not sure.
On the 89th page of Blood Treachery, one can find the Glorious Sword of Haven, a Rote designed to be used against Cainites. At the lower level, it uses Correspondence 2 and Forces 2 to teleport sunlight from elsewhere. But notice that Correspondence 2 only allows teleporting a pattern as a conjunctional effect with the aid of another sphere - the one responsible for the pattern in question. This seems to be an argument that Forces, not Spirit, is the pattern category of the thing that is being summoned in order to be used against a vampire. The counterargument to that is that teleporting a simple animal with a conjunctional Correspondence 2 / Life 2 effect doesn't require adding Mind or Spirit, yet the mind and spirit are transported with the subject anyway, and that a similar principle may apply to sunlight.
Skimming through the descriptions of the Week of Nightmares (Time of Thin Blood, Ascension), I haven't found any mentions of needing Spirit (or Dimensional Science) to reflect sunlight in a way that would make it dangerous to vampires. Allegedly it was just reflected with orbital mirrors. Similarly, vampires with the Flaw of being super-sensitive to sunlight get light burns even from very weakened sunlight (such as that reflected by the moon). On one hand, that means that the metaphysical difference between sunlight and 'moonlight' or other reflected light is irrelevant. On the other, reflection is not creation, so the argument may not be applicable either.
It seems like there is no or little explicit support for sunlight itself having special properties that would warrant inclusion of Spirit, but there is no unambiguous, irrefutable proof that it lacks such properties either. At least in the literature I encountered so far.
According to How Do You Do That, making sunlight is much harder than in the classics, and it may require Spirit depending on the GM's views on cosmology. Be advised that I have encountered multiple WoD GMs and players considering HDYDT's Sphere requirements excessive for many Effects; whether or not to agree with such an assessment is something that each GM has to decide for oneself.
As can be seen from above, the answer is rather subject to a GM's interpretation, for two reasons: first, the cosmology between editions have subtle and not so subtle differences, and second, even within a single edition, some cosmological facts are left to the GM's discretion.
In the Mage 20 companion book How do you do THAT? it says (page 32 English version, Conjuring sunlight):
At the Storyteller’s option, a scientific approach to magick could deploy Forces 5/Prime 4 to reproduce sunlight in every respect. This option depends upon whether or not the Storyteller views sunlight as a purely physical phenomenon, or as a manifestation of divine grace. Theoretically, a spiritual mage could add Spirit 5 to that Effect and then channel the spiritual essence of sunlight Elemental Mastery as well as its physical properties. In all three cases – and again, at the Storyteller’s option – such conjured sunlight might act as true sunlight with regards to the undead
So it looks like there is a chance to create sunlight of pure magic using Spirit 5... if the Storyteller accepts.
