Don't mess with life drain, it is mostly the same as before.
Spirit can be played as vitality damage mastery with vit damage staves, there are currently 3 in the game. Highest damage one is Praxidikae. You need it to try the whole thing.
I wrote some observations on vitality ternion diviner
link. Torstein159 published a video not long ago
link.
Vitality diviner is a build that can be considered top but with its staves requirement its heavily gear dependant of course. My diviner did something like 7,5K damage per projectile with deathchill, its 22K point plank. How much hp does legendary Typhon have? I don't remember the exact number, but he is the toughest boss in the game along wtih Hydra, and he died in 8 hits. Most bosses do not have that much and die is 3-5. Its like you come to the place, cast dark covenant, drop circle of power, hit-hit-hit its over. Liche is not needed there as he will always be too late. Maybe with mods like massboss he won't.
Constructs are immune to vitality damage and you should consider having another type to deal with them.