Basing necromancy damage on your vitality pool is a neat idea. Would make it more unique, instead of just another intelligence tree. Of course, it shouldn't quite as damaging as other trees if its main stat is constitution, since you're dealing extra damage and gaining extra life with one stat. Perhaps it might still require some intelligence, where you make the damage it deals a percentage of your total vitality + X-Y damage that depends on your int.
I'm not sure about changing the skilltree passive to split damage between magic and vitality. Not only do most of the skills either do physical damage or require physical armor checks (for bleeding), except infect, you tend to want to focus down armor as fast as possible. Splitting your damage between armor and health would make it much slower before you could CC them, though if you deal extra damage to vitality, that slightly makes up for it.
But the main problem with necromancy is how its most effective when you're completely vulnerable to CC. So I think it could use some skills that let you deal with this better, or at least take advantage (Percentages tuned to balance):
Blood to Bone & Spirit: Sacrifice 25-33% of your max life to gain 33-50% of your max magic and physical armor.
Shared Agony: Share negative damaging statuses (burning, poison, bleeding, acid, etc.) with nearby enemies within 8m.
Gargoyle: Petrify yourself for 1 turn, but you lose all statuses and regenerate 25-50% of your armor.
Blood Brothers: Choose any ally that heals for 50-100% of any healing you receive (basically reverse Shackles of Pain).
Flirting with Death: Deal 33-50% of the difference between your max and current health as physical damage to nearby enemies. That is, the lower your current health, the more damage you deal.
Panic of Gore (Source): For 2-3 turns, any enemies standing in the same blood puddle as you take X-Y damage and are terrified for 1 turn.
Reflection: For 1 turn all statuses that affect you, positive or negative, also affects the originator of that status (if there is one) for 1 turn.
Talent Idea:
Impatient Bloodlust: You can apply bleeding through armor, and you heal 5% of your health whenever an enemy takes damage from bleeding
