I'm sorry you bought vista, some of my customers have made that mistake and that forced me to install it on a box, which had to be brand new because if you want it to run smoothly you need some serious hardware.
you don't need to buy a new license for vista, you might have to reinstall it possibly, but i see no issue with transferring the image to a new hd, or upgrading the hardware, i dropped 2 more gigs into that box to speed it up some and had no issue with That particular deal. but as for the rest, its more of a pain than a pleasure, so i say, good luck in your endeavor, me i'd wait 4 or 5 years till they are finished with the pay as you beta test for free version.