Depends on how you value your time. If you have time to spare, buy crappy old PCs with Vista license keys on Ebay, "harvest" the license keys, install a nice shiny Ubuntu and re-sell them as Computer (No Windows) on Ebay again.

Might even be that you'll get a better price for a computer without windows than for one with vista.

Windows Vista has higher system requirements than Windows 7 or 8. If it has Vista drivers, it probably has 7/8 drivers too, they're largely the same. Windows 8 only requires 1GHz and 1GB RAM. Rumor is that Windows 8.2 may drop that to 512MB of RAM to widen the potential hardware base for low-end tablet makers.

How come it won't run on Win7 but it will on Vista?

I was under the impression that they had the same kernel and other things that made them very similar.

If it works it vista there's really very few reasons it won't work in 7 and 8. Vista is what made all the breaking changes.