July 10, 2010 is when the release candidate will be "crapping out". (The beta is August 1, 2009)
At that point I'm assuming you will need to back up your files, wipe your drive clean, and do a fresh install of Vista, or purchase Windows 7 and do a fresh install of that.
I don't know for sure, but I doubt there will be an upgrade to go from the release candidate to the official release since you'll want to be sure you have all the latest bits, and I doubt even more that there will be a way to downgrade back to Vista. It will most likely require a fresh install.
In answer to your question Yes you can go back to Windows Vista AT ANY TIME..
It is most likely that the Retail (Final) version of Windows 7 will be released long before the RC expires.. So if you want to go back to Vista instead of purchase the retail version of Win 7 once it releases than all you have to do is put in your Vista disc, boot off it than do a clean install.. (Doing the clean install will allow you to format the partition that Windows 7 is installed on and then continue with the installation of Windows Vista..
However, I would suggest sticking with Windows 7 as it offers many improvements over Windows Vista.. Not to mention it runs much better and is alot snappier.. But of course the choice is yours..