Honestly, health regen isn't particularly important to begin with. But, as far as I know, the Troll King is unaffected by vampirism. Vampirism modifies your base health recovery before Troll King increases it.
As far as clarification goes I can only assume the majority here is correct. I've seen recent posts regarding the same thing and people saying it's wasted on a vamp due to the passives
Troll king and ALL sources of health recovery ARE affected by stage 3 and stage 4 vampirism. I use it on my Vamp warden and i need to keep it at stage 2 orherwise the Recovery is wasted.