I have never used Vista. But the Lightroom specs are what they are. Apparently some people have been able to run Lightroom 5 on Vista, but it isn't supported officially and that means you won't be entitled to any official support from Adobe. That wouldn't prevent you from seeking help from this forum. You can download the trial version of Lightroom 5 and install it and see if it works. Then you'll be able to decide for yourself.
Jim Hess is being way too polite. I would say, trying to install LR5 on Windows Vista is a waste of time.
Some people do:
I've never tried it, but to the best of my knowledge, there are no known problems doing so, and it has been assumed (by me) to be a support issue rather than a functionality issue (the reason Lr5 not spec'd for Vista).
As said - don't come crying if you have problems, but do report any for other Lr5/Vista user's sakes .
To be clear: if at all possible, it's best to upgrade your OS instead, but if not possible..
Thanks for the replies - think I will try the free download and see how it goes