I like Vermaelen but think we will end up selling him this summer. Personally I think he could make Utd stronger despite what people think of his form in a Arsenal shirt. I think it's fine that he wants to go somewhere where he has a genuine chance at being first choice. He's had his opportunity here to do so and if he can't, especially at his age, then for his career, he should be looking to move. Unfortunately I think most people agree that we have enough areas needed to strengthen without having to contemplate a 3rd choice CB.
As far as potential replacements go, I think we will have to look at those CBs who are not considered genuine starters ahead of either PM or LK. I would imagine someone young with more raw potential experience than talent from somewhere in England, France or Germany or someone slightly older from the "lesser" leagues - i.e. anyone willing to bide their time as third choice. If we were to negotiate with Utd, I reckon we may look to either Phil Jones or Chris Smalling seeing as how we have held serious interest in both before.
I do agree we have other priorities without adding a new CB to them, but still feel that we should sell him. The farce over the captaincy would be settled and we can possibly try for an option that could complement LK and PM if one is injured.