It doesn't invalidate the serial for Vista, it just upgrades the OS. You can reinstall Vista later if you'd like.
Note that the upgrade product key for Win 7 will only work as an upgrade though. You can't do a fresh install with an upgrade disc without having the physical media for a qualifying version of Windows. If you have one of those large recovery sets like Sony is notorious for making, or if you have a built in hard drive recovery like Dell has and didn't spring for the actual media you can't do a clean install.
With Vista (and I assume this still applies to 7 as well) you could install Vista without putting in a product key, allowing for a fresh install. Then do an in-place upgrade from fresh Vista to fresh Vista. This will give you a clean install of Vista without having previous media available.