I'm willing to bet that if I punched every bit of a vampire out of a vampire so that they were 100% plant it would be an entirely new species. You couldn't do it with a human, because the process would kill them. Vitalogy doesn't put peoples blood back inside their bodies, I punch out a heart and they're dead in moments and you can't bring people back from the dead.

But with a vampire the only tricky part is taking apart their brain piecemeal so they're always just grievously injured instead of dead.  After that they wouldn't be weak to the sun anymore, because they're not a vampire anymore, but they'd still be able to do all the body horrors stuff because the replacement parts would replicate the perfect control of their biology that vampires have. The ultimate, though warped, evolution of the Plantae kingdom. A plant that could think like a man.

This is all thought experiment, or course, because Rodger isn't the villain of this part and is more likely the guy that dies horribly near or during the final battle.