stefor:
+1 Because you owned the copy of Vista at some stage doesn't entitle you to the upgrade. YOU have to own and have Vista to legally get the upgrade.
As the OP stated - windows 7 upgrade can upgrade from xp or vista so as long as you already have that on your target computer you will be fine.
So basically he is saying that the upgrade isnt directly linked to the copy of Vista that was bought to get the upgrade, but any legal copy of Vista/XP can be used to perform the upgrade with.
Therefore if you own Vista already - you buy this then sell the second copy of Vista or even sell the original version and then use the upgrade yourself with your "2nd" copy of Vista.
Should be fine in theory and you get the upgrade for what you pay minus what you can sell one of your (now) 2 copies of Vista for.