I think that United will be looking for more than just two defenders to replace Ferdinand and Vidic. The other defenders they have are poor and they would probably look to replace one or two of them too.
Put yourself in Vlaar's shoes, regardless of how the rest of the World Cup goes for him and Holland. You can go to a club that's one of the top teams in the world, play alongside fella countrymen and for your national manager and maybe get a nice contract that pay good bonuses and a salary of around £50,000+ per week. Or, he can stay at a club that fights relegation season after season with an owner who clearly wants out and, surrounded by very average players. He may be on £30,000 per week salary with an improved offer of £45,000 in the pipeline, who knows?
He's at the age where he probably has just one more move remaining in him and I can't see him staying faithful to Villa if he was given the chance to join United or, any other top club for that matter. I certainly wouldn't blame him for moving on either, as the future of Villa is completely in the lap of the Gods.