The last version of Photoshop to support Vista was CS5.
CS5 is no longer for sale so if you don't already own CS5 and want Photoshop then an operating system upgrade is your best option.
Alternatively Photoshop Elements 12 still supports Vista SP2.
Even though windows vista is not officially supported, the perpetual license version of photoshop cs6 should work on vista.
Certain features may not be as fast as windows 7 or 8, but should still work.
You could download a trial and see:
Photoshop cc (version 14) will not work on windows vista.