Re: communication not available

Unstable network sharing is usually more of a problem when peripherals or drives are on the Vista machine. I have sadly seen many Vista installations where file and peripheral sharing one day stops working although no sharing/permissions settings have changed.
What may be happening is that your Vista wireless is not connecting properly to the router, also a too common issue in my experience.
Although other Vista and XP wireless machines may have stable wireless connections to the router some Vista units periodically cannot connect without resetting the router. I do not know if the blame lies with Vista or the wireless hardware/driver in the computer.
Presumably you have rechecked the XP settings for the printer and they are correct?
Try resetting the router (unplug/replug the power) and reboot your Vista machine. If it connects stably to the network (you can share files with the XP machine) but you still cannot share the printer then delete and reinstall the share on the XP machine.
Hopefully you will re-establish printer sharing. However it is a sad fact of Vista life that for many users the sharing will not last . . .