A few interesting anecdotes:
Fighting 1v2 against a torchbearer and Erebus, at about level 9 on Cataract. I start in against the TB since he has less health, and more damage output, when the Erebus teleports in to help. Normally a 1v2 situation is doom for the player... but not with Sigil of Vitality!
I spit on the TB, turn on Ooze, grasp him, and as soon as the grasp starts, I activate the Blade of the serpent, completely refilling my mana. I start meleeing TB, and he's now taken about 2200 Damage, and has 800 hit points. He has fireballed me, and activated a circle of fire right after the grasp, erebus and his minions are attacking me.
TB starts running, and I choose not to spit on him again, saving it for erebus. Finish TB off when I've lost about 3000 health (from 5500 to 2500), still standing in his circle of fire.
I turn and spit on Erebus, most of his minions are dead due to ooze at this point, and he's got 3500 health to my now 2000 health.
At 1k health, I pop the sigil for the insta heal of 2500 damage, right before he deals a basically lethal bite to me. Since I've still got a lot of mana (Blade of serpent has been refilling for most of battle) I get another grab on him, then a spit as he runs away, for the double kill.
Afterwards I had about 1500 health. I would have died easily had I not had the sigil.
This isn't intending to prove anything about the item or build necessarily, but I thought people might enjoy it.