Voldaren Ambusher's ability cares whether an opponent lost life this turn, not how their life total changed. For example, an opponent who gained 2 life and lost 1 life in the same turn still lost life.

Voldaren Ambusher's ability counts the number of Vampires you control as it resolves. If something happens in response such that you don't control any Vampires when it resolves, Voldaren Ambusher won't deal damage to the target creature or planeswalker.