Whether or not you are an enrolled student, you can take summer courses at UVA. If you are admitted, we highly recommend that you come in and take at least one class. This gets you acclimated to the University, our teaching styles and gives you some time to become accustomed to UVA before the hubbub of Fall semester. This is not at all required, but just a suggestion. This will not hurt or help your chances if you are on the wait list. Your spring grades are the most important factor when we look at the waiting list.
If you decide to take summer classes at your college or university, these credits have the ability to transfer if there is an equivalent at UVA.