Vena assumes he understands. He assumes his own understanding of a lot of things and though he's been around long enough that he's right often enough, this is an entirely new situation that's been manufactured by an entity that he still feels like he only has base level knowledge of. But the way mention of Su-jin eating the freshly dead was tacked onto the end of the short explanation makes it seem like it's an absolute last resort. A sort of fail safe, should Vena make a mistake that he's expected to not make. And he can't imagine his previous arrangement with The Council is meant to continue now that they've found him a more permanent solution.

Which makes him feel safe in his own assumption that the intent is for the two of them to use each other more directly.

When the cloaked figures move to leave, Vena actually turns to look over his shoulder to watch them leave, properly acknowledging them for the first time and the last. Su-jin's words quickly draw his attention back, however. "You will." Next to poor Su-jin, Vena looks like the picture of good health. He doesn't deny himself a meal like Su-jin does. But the deaths of his victims also aren't quite as quick as Su-jin's, nor are they quite so obvious. They're not even guaranteed. He has the option of simply finding a new victim before the previous one passes. He can imagine Su-jin has the same luxury.