I think you should consider a couple of things: which skills you absolutely want, and how many points to put into them.
Take devouring swarm: the resist reduction is very good, but it doesn’t scale well beyond the soft cap (above 16/16). It’s a very good skill also because the damage over time makes devotions proc very easily (a trait in common with grasping vines).
Grasping vines one of the very few crowd control skills you have. I don’t know how good it is damage wise, but not very much I think.
Storm totem provides only extra damage, it could be good, but I don’t know if you need it. It’s another good skill for devotion procs though.
Wendigo totem is similar to storm totem, but it heals you in exchange for dealing less damage. Good utility, not sure if you need more healing if you factor in drain essence and syphon souls, this looks really like a potentially immortal build if you can avoid getting oneshot.
On the Mogdrogen’s pact line consider heart of the wild up to 10/10, the extra health can never hurt, it may also be key to your survival, especially in the new content in ultimate. Oak skin is decent if you really need the pierce resist.
Drain essence, passing to the necromancer tree, could be a candidate to be the main attack, only I’m not sure how the damage is, I personally found it lackluster early on in the game, for the amount of energy it needs to sustain, but that could be better later in the game, with more energy regen and %vitality and aether. You will need to build some energy regen to sustain, I don’t think 50ish will cut it, but it shouldn’t require as much as aar from arcanist. For hungering reach on that line, I think I wouldn’t overcap it, those extra point could be more useful elsewhere. The damage lines should have as many points as possible to get good damage.
Syphon souls is pretty straightforward, I’d put as many points as possible and I like a lot the offensive ability debuff on blood boil. Depending on whether you care more about the damage or the debuff, you could take the trasmuter for 100% uptime, could be useful for bosses despite getting healed less.
Spectral binding I think would be a good source of both health and offensive ability. The buff to aether damage is a good extra, but probably not necessary (85% at level 12). Spectral wrath on its line is good, resist reduction is always good, I’m just not sure about the range, it feels short, but then again you will most likely find yourself not using the full reach of the other abilities unless you’re godlike at kiting, which still wouldn’t be optimal having to channel drain essence, and your main survival option should still be lifestealing through your abilities, while having enough health to actually do it (so not dying in one or two hits).
Now the one ability I think you should always have, and maybe even cap (though getting it to 6/10 should be good enough, beyond that it doesn’t scale very well): mark of torment. I think it will be a very very good “oh shit!” button, and it’s the only one you have with this class combination.
Bone harvest: I’m not too sold on this skill with this build. It could help giving you some extra aoe, and the buff on soul harvest is not too bad, but I think in the late game it’s not going to be very noticeable. If you like it though, I suggest putting one point on dread for the extra range.
Ravenous earth. It doesn’t really belong with the build you’re doing as a theme, but it has decent potential. There is some vitality damage, but what interests me the most is the resist reduction on foul eruption, it’s flat, so it would stack with the percent one from devoring swarm or spectral wrath. The poison damage on the main skill makes it good to proc devotions too.
Harbinger of souls is a good aura. I’m not sure how worth it would be to overcap it, but if you happen to have the points to spare go ahead, the cast speed is very good for drain essence.
With all the stuff there is, I’d love to have double the skill points 
