According to this site:
http://support.microsoft.com/kb/932087
Office XP is "supported" under Vista. However, any issues that do occur will
not be fixed, since Office XP is no longer being developed (which makes one
wonder what "supported" means).
I have installed various Office XP applications under Vista and have not
experienced any problems, however, except for problems that occur when
running under Windows XP, as well. I.e., I have not experienced any Office
XP problems that are due to Vista per se.