Well in such case do not get surprise if some of your "Users" and "ProgramData" would get incorporated back as it was before.
Before such an upgrade I would make an image of the whole Vista installation.
Acronis True image can create such a file that can be mounted later on as a data file.
Or, download this and run it on the current Vista installation and make a VHD file from the whole drive.
http://technet.microsoft.com/en-us/sysinternals/ee656415.aspx
Store the VHD on whatever you have that can store such a File (do not store it on the drive that you intend to use for Win 7).
Do a clean install of Win 7.
When done copy the VHD file as a regular file to the Drive with the new Win 7.
When you need anything from the old Vista drive log to Win 7 Computer management open the Disk Management, right click on it and mount the Vista VHD as a virtual drive. Copy from it whatever you need.
Or the You Tube way,
http://www.youtube.com/watch?v=m3Pxn23dFuQ
.