Vampire has never been seen as a passive ability; in fact, Liquid Shadow replaces the current active ability, and Momentum, though an active status and thus removable, will create an active ability on an ability-less creature. I doubt putting two actives on one card is a good idea.
Alternate idea: perhaps this creature could begin with only Vampire, and when targeted by the Heal ability or Holy Light spell, would invert its attack? That would not require an ability, active or passive, just an edit to the card text - and it would create more use for two aspects of Light that are currently less commonly used.
