If your computer is made of uefi hardware, then i don't think you will be able to intsall lower versions of Windows. I tried to install Win 7 on my Uefi Hardware Laptop But I ended up with "no OS found error" So If your PC has Uefi hardware I reccomend don't install Vista. Because result are likely to be similar as mine. If you don't have uefi harware you can easily install Vista by Formatting the C: drive while installing Vista.
If you don't know what UEFI is? simple ans. If you have already Windows 8 on your pc and it has Glowing stickers showting Windows 8. That's when you have an UEFI harware.
Note- Someone please correct me if I am Wrong about abyhitng posted in this whole answer.