Well you have two options.
you can dual-boot, but you'll need to download another program that gives you the choice of which operating system you want to start.
You can then save all your files from Vista.
If you only want to run windows 7, you will loose are your programs that you have installed on Vista, but you can always copy your important documents, picture, music, movies, etc to a external USB drive or a large flash drive depending on the size of the files you want to keep.
Along with 99% of the regular posters here, we never recommend you do an upgrade. A 'clean" install is the only way to go. You'll end up with hundreds of useless files left over from vista which are useless and just take up unnecessary space.
So I'll let you figure that one out.
this will probably be the end of my posting for the day, so i'll see you tommorow.