I'd question using the word "Legal", since EULA click throughs have never been determined to be legally binding on the end-user.
In other words, the EULA has the restriction written into it, but there is nothing technically preventing you for doing what you want with the software once you purchase it. It's more of a way for MS to maximize their profits.
First off, all the "eye-candy" on the higher priced versions is useless in Parallels. For the best speeds, you want all that eye-candy turned off anyways, so why pay for it.
Personally, I don't know what all the big worry about Vista is anyhow. WinXP SP2 in a virtualized environment is more that adequate to run ANY Windows program out there and run it well. Does anyone think that the program will run any Better or Faster using Vista?