It's true that Vena is not often denied what he wants. People have a remarkably hard time telling him no. Humans have no real defences against him, nor do some monsters. It can take a little longer and require a little more effort on the part of the incubus, but even then, he usually gets what he wants in the end. He's sure he can count the number of times he's been denied.

So of course he doesn't allow Su-jin any escape. He doesn't allow him to put distance between them, to escape his influence, to stop breathing him in so completely. Besides, Vena has no concept of personal space (not when he'd far prefer to have people in his personal space). He deals in closeness, after all.

"Don't underestimate my ability to take control should you lose it."