The concept is something I've been wanting for a long while - it really speaks to my inner basterd who just wants to cackle madly as his foes melt down into puddles of goo... (Waddayamean, it's just me ? :p )

I'm trying different builds right now to see if it is balanced. Some remarks, since there's no forum thread for this yet (take with a grain of salt, those only imply my personal opinion) :

- Poison Cloud scales pretty poorly for such an iconic power, AFAIC. Even with the stacking effect, it just FEELS much weaker than a Oozer's Poison Spores. Granted, we get it at level 0 instead of 4, but still. Maybe give it the ability of applying different poison types with additional levels (say deadly at level 1, numbing at level 3, and Insidious at level 5+, to keep it from being too powerful), or the same kind of crit bonus PS has ? Considering how many things are poison and/or diseaase resistant/immune in this game, it really needs to pack a bit more punch to justify investing in it.

- I have no idea why the class description emphasizes Willpower. Not a single one of the trees relies on it, it doesn't directly benefit the vim pool in any way, and the kill bounty bonus is too laughable to justify the investment (1 extra vim per TWENTY will ? Shia right...) I'd probably switch it to Cun - with DoTs, crit bonii can stack pretty damn fast.

- I get the feeling that Blood Feed is meant to be the main on-demand Vim source for the class (correct me if I'm wrong) since we lack both blood casting and drain. The idea is neat, but impractical : being stunned, dazed, silenced or confused has a good chance of preventing the use of the spell, yet it DEMANDS that we suffer some kind of impairment to work at all. So it's a toss-up between hoping it'll be useable, or finding it blocked, and clearing the status with an infusion... just to see it become ineffective.
I like the idea, really I do, but I've found myself short on vim AND unable to use this one too many time to buy into it. Same thing with Decayed Harvest - it's neat, except that when you've been manaclashed to zero Vim, it's kinda hard to find ways to disable AND kill a critter to recoup. Chances are, they'll disable and kill US first. A Defiler caster class really needs an at least semi-reliable way to regain some Vim.
I'd suggest maybe changing Blood Feed to draining our life into vim based on the number of impaired (or hurt) enemies in a certain radius - a kind of gamble : if you're neck-deep in critters, you could regain a LOT of vim... Assuming they're under status effects (or hurt), and you got enough life to take it. Or, heck, since it's supposed to "feed on our own suffering", just make it so it'll give us Vim based on the damage we take ? I dunno, just a thought. But as it is right now, I'm not sure it's really useable.

- Blinding Spray and Scythe Beetle starting at a radius of 2 means that some moderate to heavy point investment is needed to make those spells practical. I don't think upping the starting radius to 3 would be too crazy a power boost, frankly.

- EDIT : I just noticed that Vapour Form actually poison me, whereas Poison Cloud doesn't - which is weird since it's a Point-Blank, defensive spell that's supposed to protect me... All the more since there's no way to escape the effect, and a stacking 5-turns poisoning on a low-HP class is plain murder.

- Second EDIT : Poisoner's curse causes a LUA error. Where should I send it so HousePet can see it ? I feel I should specify that it WORKS, it just causes an error pop-up to appear.

Again, those are just my opinions, and I want to reiterate the fact that I really, REALLY love this class. So thanks, HousePet. :-p