If the point is to synergise with rage potion, I'd increase its HP to 6, or even 7.
Simply because, yes, whilst it heals a lot, Voodoo is both a lot quicker and harder to kill with (damage-based) CC in the first place.
Voodoo: Play it. Rage it. Boom.
Sinister Vampire: Play it. Wait a turn to activate ability. Wait a turn to get 7 HP. Hope it wasn't CC'd at all during this point. Rage it. Boom.
Or, even, compare it with Guardian Angel: Play it. Rage it. Self-heal. Boom.
I'd make this ability merely active, not an activated.
"Whenever Sinister Vampire deals damage, it gains that much max HP."
If you want to have the vampire still have a link to fire:
"Whenever Sinister Vampire deals damage, drain
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to have it gain that much max HP."
Oddly enough, it makes a strange and very interesting combo with overdrive + gravity pull, but that'd require a trio.