If you use Siphon souls and BH you can go vitality build,for example choosing Shaman as second class.For aether-Arcanist or cold-Reaper. So depends on the prefered damage type.
Those are pretty good suggestions, though I still have no idea whether to take Ill omen or Reap Spirit.
I have been seriously considering Shaman, but heard that ritualists don’t have much in terms of end game gear so I’m kind of hesitant to go that route.
Ill Omen sucks. Don’t take it. For damage reduction take Ravenous Earth. Siphon is mainly a debuff and a heal - you won’t be dealing much damage with it. Currently the biggest damage output in nerco skills is Ravenous Earth (maybe challenged by Drain Essence). Check it out and see if you like the play style.
If you want nerco to be your main mastery, and you like its skills, then the best support would be occultist, oathkeeper and shaman. Then you can focus on Ravenous Earth with any class. Bone Harvest is better on a ritualist or oppressor with Blood Knight set. Ravenous Earth has the advantage that it’s supported nicely by Dark One cabalist set that is target farmable. This is for vitality.
You can also try cold Bone Harvest with a reaper (imo weak).
Or you can go for a Drain Essense build in many dmg types with many other masteries.
Most of the stuff in my guide applies to the Oppressor as well regarding gearing, game mechanics and farming Dark One set etc.
Bone Harvest you do have some faction gear support (shoulder from Barrowholm etc)
This is something I have been fooling around with https://www.grimtools.com/calc/YVWlLzJZ for a potential beginner guide. Damage is fine, but hard to get OA, DA and Energy sustain
Perhaps you get some ideas at least