Have you contacted the vendor to tell them of the probelem and have they confirmed that they installed a genuine version of Windows Vista? That is the first thing you should do. If not then it sounds as if you have been sold a pup and need to buy a genuine Windows Vista key which you can do so from Microsoft - just avoid any dodgy cheap keys you may see on line. Invariably they are fakes too and will show up again in 2-3 months.
You might want to read this from Microsoft