My level 36 Witch Doctor is relatively effective against big groups of monsters, but I have problems defeating some vampiric bosses. The problem is made worse by the gargantuan which is delivering life to the vampiric bosses.
If I play with zombie-dogs I have absolutely no chance to defeat the boss.
You need a better weapon. All of your summons' (Zombie dogs, Gargantuan, etc.) damage is based off of your weapon damage. Better weapon means they do more damage, and they should be able to counteract the vampire.
If the boss ALSO has Desecrator, Plagued or Molten, you're in for some fun. The damage from these patches also heal it. Try to keep your pets out of them, and most especially, keep yourself out of them, too, even if you don't notice yourself losing health. Quite often, you will be standing on several patches (especially Plagued), and each one will heal it.
If you use Haunt, you can probably do enough damage over time to negate the gain that they get from Vampiric. You then just need another source of damage to wear them down with. This could your primary attack, or something like Spirit Barrage. Having Pierce the Veil will help a lot too in terms of increasing your "burst damage".
Don't worry too much about them gaining life from attacking your pets, they'd gain life no matter what they attacked, you just need to do damage faster than that.
I use spirit walk, haunt, and locust swarm against vampiric. Very hard to kill but doable. No pets. enter spirit walk, use haunt and locust swarm, run around. and repeat.
Yesterday in nightmare i had a vampiric plauged rare monster. I had to dismiss my followers and pets and use Hex to turn him into a chicken in order to stop him healing. At the time i had a stupid quest follower, who i could not dismiss. Tyrial or how ever you spell it, and because of him i was unable to kill the monster. But the idea still works. Hex to prevent him healing, grasp of the gead to slow him down. acid to deal the damage. bg bad voodoo to increase damage.
