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..