How can I downgrade the OS on a Mac to the original one it came with? If I buy a mac with OS X 10.6.5, and then I upgrade it to OS X 10.6.6, how can I downgrade it back to OS X 10.6.5? I don't mind having to reinstall the entire operating system if necessary.

Could this same method also be used if I upgrade to OS X Lion (OS X 10.7)?

The reason I'm asking is because I heard that OS X 10.6.6 removed support for installing boot camp using a Vista CD, and that's all I have. Thus, if I ever want to wipe my computer and reinstall everything, I want to be able to still use a Vista CD to install boot camp.
You can reinstall the OS back to the same version that it shipped with by using the restore discs that came with your Mac to wipe your disk and reinstall Mac OS X.

However, just because Apple has officially dropped support for XP and Vista with Boot Camp doesn't mean it will stop working - the most likely situation will be that new computers will have hardware that Apple will only provide Windows 7 drivers for. Existing hardware should be unaffected.