As long as you re-install the XP license onto its original PC, you should be able to sell the Vista Upgrade package. If you kept the Vista Upgrade installed, then the XP EULA would bind the XP and the Vista Upgrade licenses.
Resale of "Retail Upgrade" packages has never been disputed by Microsoft on sites such as eBay. And you presumably have some sort of COA or licensing paperwork for your XP license. So I believe that, in this case, you still own a separate XP license and should be able to use it within the terms of the original license.
Yes, as mechBgon points out, it's a grey area. But it seems to be a perfectly "fair" interpretation of the EULA. Practically speaking, once Vista is removed and XP re-installed, there's no indication that the two were ever bound together except in your memory.