I see Vincent as more of a Hufflepuff than Slytherin for sure. He's cunning and willing to do anything to get what he wants, but he doesn't do that for personal gain at all. Everything he does is to protect Gil and he's fiercely loyal, but it's packaged in a less morally sound character than you'd expect of a 'puff. I think he could definitely fit in well with the Slytherin crowd, but at his core he's a Hufflepuff.
I agree with this! :) Though, I think he does do it from a place of selfishness, though not necessarily through a place of personal gain.
I was debating back and forth on which one to place him in, because I do think that he has a certain Slytherin aspect about him but that his motives are much more Hufflepuff in nature, like you stated. I think I might actually go and switch to him Hufflepuff because I feel it does fit him better.