Ah, but that's the thing about Vates... odds are he won't want to keep Thanh as a slave, not really. He's a Feral and they're used to being discriminated against, ostracized, and taken advantage of when people aren't outright massacring them out of blind fear. Plus, Vates isn't the type to force anything on people, mostly because he doesn't like things being forced on him. If anything he'd use the whole "slave" angle of things as a way to protect the big guy in secret, keep other people from taking advantage of him while treating him quite decently. Probably even remove the enchantment on that slave ring 'cuz, heck, he can.
Entirely up to you if you think that kind of relationship'd work out, of course.