If they use their last life to play an action modifier, the action fails. If they use their last life to play a strike, combat ends before strike resolution.
Allies who can play as a vampire
22 cards
Rulings
If they takes aggravated damage from a card they plays as a vampire (eg. Burst of Sunlight), they lose one life but do not go to torpor.
If they use their last life to pay for an action, the action still resolves fully. If that leads to an oust, the oust is resolved before resolving effects triggered by the fact that they burn.
If a card they play as a vampire sends them to torpor (eg. Reform Body), they burn. REVERSAL of previous rulings.
They cannot use a card to commit diablerie (eg. Amaranth)