Pretty cool class, I must say. Lots of cool stuff, and it really comes together to convey the concept well.
My only major critique is that this should probably have something to address the possibility of other methods of becoming undead being in the game. I imagine even if I hadn't made an undead species, somebody else would have eventually, or if not a species perhaps a Prize, a different class, a specialty, or a Species feat.
The class already treats undeath as a prize -- which when you think about it, is probably by far the best way to handle access to the type. Whether it's a curse is dependent on setting and what you accompany the type with but by and large it's going to be woohoo time for a player when they gain it.
Getting a prize slot back is an option if you're already undead I guess, but the class isn't really intended for the already undead, it's for situations where undeath is unusual and dramatic.
Even given awareness of this class, I imagine sooner or later people would start to get tired of the only option for playing an undead character being a vengeful combatant class and want a way of playing other kinds of undead without relying on multiclassing and eating an action die penalty for doing so. In fact, come to think of it, I've heard that the Reaper class in Spellbound will make you undead...granted, I don't imagine that multiclass would be at all a good idea, but this at least confirms the general principle that it's not realistic to assume this class will always be the only option for gaining the undead type.
Reaper doesn't grant the undead type until level 11, which is a hell of bigger investment than anything the Revnant is asking in return for giving up an AD if you want to multi-baseclass while undead.
Technically you could pick up Risen
at Sage 4 using cross-class training if the GM was okay with the idea
So, yeah, there should either be an alternative level 1 benefit for if you're already undead, or a "non-undead" prereq to the class (possibly also coupled with some prohibitions on gaining the undead type later or something).
As noted above, not so much. It's not like the GM has to let a player enter the class if they don't feel it fits.
Also, just curious, any particular reason why plants can become dhampires after level 1 but nobody else can?
No idea. I may have just cut'n'pasted a feat structure and forgotten to edit the old text out.