I cringe at Vista and avoid it like the plague... but if you must downgrade, If it were my computer, I'd install a removable drive bay which allows for 2 Hard drives to be installed into removable drive cradles and have 1 drive with Vista and the other with Windows 7. This gives you the best of both worlds for under $100 if picking up a $65 SATA drive and 2 x SATA Removable Hard Drive bays @ $14 each + shipping and installing 1 of the 2 bays and installing existing Hard drive to 1 cradle and the new hard drive to the other to contain Vista.
2 x
http://www.newegg.com/Product/Product.aspx?Item=N82E16817999024and 1 x
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148764If you Downgrade, you will have to be wiping out all information and installing clean Vista from restore set. You will need to backup your important data. Licensing if you went with my suggestions of 2 drives is legal because only 1 instance of Vista or Vista/Win7 upgrade edition will be used at a time.
Only downgrade I have ever done was to get rid of Vista as soon as the computer was taken out of the box new and installing Windows XP Pro to it.
I use these removable drive configurations on my systems that are multipurpose. I can isolate projects by labelling the drives and pop one out and the other in and keep data seperate etc and run different environments quickly. Other alternative to this is using Virtual Machines, but for some projects its best to work with real physical machines than a virtual system within another physical system running concurrently.
