It can be quite possible that you will have trouble playing the Closed Beta with Vista. Nevertheless they are working on a patch that will allow Vista users to play without problems in the future. Perhaps if you are very lucky the reason they delayed CB might be the patch to let Vista users try out the CB as well.
If it turns out you can't play the CB with Vista then I suggest you keep lurking because I am positive that they will take care of it soon. It's obvious they have been testing different systems to make sure everyone would be able to play.
Good luck! n_n