Having been using this skill recently, I felt it was noticeably underperforming, and wanted to play with some ideas on why and how might be helped. The numbers are taken from the wiki.
The passive effect of the signet heals you for 325 when you’re hit at level 80, with a one second cooldown. The active effect marks an enemy for 6 seconds with 25 stacks of an effect that makes anyone who hits it siphon life from them (while stripping one stack), with a siphon damage of 201 and heal of 392. This active drain has a one second cooldown as well. In addition to the active effect, using the signet gives an initial heal of 3,960.
For comparing one healing skill to another, the most important factor is naturally how much health it gives you, versus the cooldown time. Because of the 1 second cooldown, the most the player can hope to siphon from their target is 6 times, combining with the initial heal for a total of 6,312, with a cooldown of 35 seconds. This compares to Well of Blood (initial plus all pulses) with 6,640 and 40 seconds, Consume Conditions with 5,240 and 25 seconds, and if one were to use Summon Blood Fiend and immediately consume it, 3,960 and 20 seconds.
While this looks comparable on the surface, the context is where the signet falls behind. The drain assumes an ideal situation where you get the maximum number of hits, and only goes down from there. Consume Conditions can heal dramatically more if you have multiple conditions on you (which is often going to be the case when you need a heal), with the substantial bonus of removing those conditions, with a faster cooldown. Well of Blood has substantial trait support including reduced cooldown, added protection and health siphoning, and the possibility of being cast outside normal use via reviving. Both of these skills scale considerably better than the signet with healing power, as well.
Blood Fiend is harder to compare, but with a passive heal of 926 every 3 seconds, the signet requires the necromancer to get hit every second for comparable numbers, which is going to lead to death pretty quickly. While the minion can be destroyed, it also has the fastest cooldown. The only necromancer heal that can’t have its cooldown reduced with a trait is Consume Conditions, which is still going to recharge faster than the signet anyway.
Its passive effect means the Signet of Vampirism basically functions as a strange, overcomplicated damage mitigation, with some damage returned as health instead of reducing the damage. However, its low rate (barely helped by healing power) and internal cooldown means it’s damage mitigation that becomes less effective the more aggressively the necromancer is attacked, which is when it’s needed most. Slow, weak hits are not likely to put the character in much danger in the first place. The active effect provides moderate healing at best if you scrabble for it, with a long casting time and cooldown. The exchange for this is that the target-marking effect lets a party all heal from the victim. While party-aid is an area the necromancer is notoriously bad at, sacrificing your own survivability in exchange for a very modest group heal (scarcely over 2000 hitpoints for an ally, assuming the timing works out for them to take advantage in the first place) isn’t a very appealing option for a class whose survivability is supposed to be a key factor.
I understand not wanting to turn the active effect into basically a duplicate of Litany of Wrath, but there are other ways the skill could be improved. My suggestion would be to turn it more toward that survivability, where it suffers badly; a healing signet that doesn’t heal you at all while passive is a problem. Some ideas on what might be changed will be below, since I apparently went to long on this one.