I have heard of a rule with no quotation given (so presumably a house rule) that implies that 3+ health levels worth of lethal (or aggravated) damage dealt in one attack may severe a limb, or simply render it useless. This can be indirectly supported by the rules for regrowing a limb, such as the one that ghouls use (from page 501 of V20 Corebook, page 501).
Grave's Decay path of Necromancy level 3 power, Wither, implies that 2 levels of aggravated damage are enough to render the limb useless, but that may possibly be just a case for that particular Discipline.
Are there any WoD rules that allow severing limbs by non-supernatural means? If not, what other passages of official books may imply a possible house rule to manage that?