Yeah, we're good with that. Minor will-o-wisps won't give a whole lot, but if you get your hands on something big and juicy, that might be another story. Of course, if you get your hands on something terrifying and wind up dead, that'd just be poor form.

To note, though. Let's say you're critically wounded and on your last legs and start vampirically sucking up the power of the shadows. You're still critically wounded, your next attack might just be stronger. In fact, after expelling that energy, you might even be in worse off condition than when you started, so choose carefully.