The base question is whether it is a problem with the DVD or a problem with your optical drive. First, verify that you have a DVD-ROM drive and not just a CD-ROM drive...the latter cannot read a disc meant for the former.
With that established, boot to Windows XP, insert the Vista DVD, open My Computer, and see if the icon of the drive changes or remains the same. Try double-clicking on it and tell us exactly what, if anything, happens.
After that has been jotted down, remove the Vista DVD and try inserting any other DVD (not CD) you may have and see if your computer is capable of accessing it.
If you can access other DVDs but not the Vista one, I would return it to Best Buy as being defective. They can't refund your money, but they can exchange the DVD for another. If other DVDs fail, though, I would bet that either it is just a CD-ROM drive or it is a DVD-ROM drive that is going bad.
Let us know what comes of it...you may get lucky and it turns out to be a missing driver or a loose cable, but post an update so we know what path to go down.