I presume it’s attached by USB cable to the XP machine, i.e. it’s not a true wireless printer!

I can only guess but I suspect you have installed it on the Vista machine using the USB cable. You have a choice of leaving it as it stands or removing it in Vista, then adding it again. I can't advise you as you may have had a lot of problems getting it to work initially.

However, to remove and add it again:
Turn on the XP machine and the printer then on the Vista machine go to Control Panel > Printers, right-click the printer and Delete. Now click Add Printer and follow the wizard to add a network printer.