No, I want to keep Vista and have 8 too. When I've tried Ubuntu with Wubi, it allows me to choose which OS I want to use on computer start-up.
You can boot off of a VHD, which is a virtual hard-drive (a file that sits on your hard-drive). This allows you to install Windows without having to create separate partitions or have another drive attached.
A tutorial on how to do it can be found here
; it was written for the Developer Preview, but should work exactly the same for this version as well.