I see you made it over here, welcome. It should give you the option automatically after you make the partition and install 7 on the new volume. When you boot up, it will have a screen that says select an OS. 7 will probably be on top and then Vista and you would just press enter for the one you want to boot into. I would still use at least 50-80GB though, you can have it till August so you might want to install a lot of things to try out and test.