The worst-case scenario would be Vince poking around on his own. He knows Draco spends most of his spare time in the Come and Go Room, but not what he does there, or even what the room turns into for him. He doesn't think Vince is smart enough to figure it out, and nobody else knows what he does in there either, so there's no one to ask. But if Vince does start trying to get in by himself, it endangers not only the mission, but he and Hermione.