What is the size of your present hard drive?
If you have had no problems with 7 Beta, why not hang on to it for a while. There may be a solution .-it is only a Beta.
What about shrinking your Windows 7 installation, and making room for Vista on another partition? You will then get the best of both. Installing Vista will automatically give you a dual boot option.
You can shrink, if you have room, by opening Administrative tools - Computer Management - Disk Management.