Yes, you need a vapor barrier. But, depending on how the slab is currently insulated, you may have a vapor barrier. Make sure you don't end up with 2 vapor barriers. When I finished my basement, I firred out the walls with 2x4's and insulated with kraft faced fiberglass insulation prior to installing dry wall. (The kraft paper goes toward the heated space)
By the way, don't be surprised if you don't get back what you put into your basement. I talked to several realtors prior to finishing my basement and they suggested that we do it because we wanted to and not for what it would do to the value of our house. The rule of thumb here is it does not add much.