I am a little worried about this..

If Dell told you that Win7 would auto-activate on a Vista machine then they most likely told you wrong. It would be the first time we've ever seen that here and we've seen everything. They can't even legally sell a PC with Windows Vista or 7 pre-installed on it unless it has a COA sticker certifying it is Genuine. So if you don't have a Win7 COA sticker then you don't own Win7 on that machine, at least not from Dell.

If Dell continues to maintain that Win7 will work on that PC then they should be willing to ship you a Win7 Reinstallation DVD. If not I'd ask them to send you a Vista Reinstallation DVD and do an install similar to Clean Reinstall - Factory OEM Windows 7 which will make Vista nearly perfect. Dell is the only maker which still ships these clean-copy OS DVD's with only the OS, pre-activation and Dell branding. Is that what you used?