My guess is that SOMETHING was not included on the external when you saved to it in XP. Did you try "SAVE AS"
Another thing that just struck me.... Is the drive letter on your external the same in BOTH xp and vista? If your external is drive M in vista and its drive K in xp (where you ORIGINALLY did the save) then when you hit the save button it tries to save to K but it can't because there is no K drive. (This is just a guess though)
My wife's laptop has Vista Home & my work comp has XP Pro, I regularly work on one comp with the external & take to the other w/o issues.
I wouldn't say there should be an issue with saving though. There will be warnings upon loading that files are missing but shouldn't be any issue saving.
Thanks for the replies. I am using 'Save as '
The error message reads -
"An error occured while saving the project file (name of file).veg,
The file could not be created."