The way it sounds is that, no, you can not install that (OEM) version of 7 that you have from your old system on another system. Well you can for the 30 day trial only,
IF it will let you do that, but you can not activate it. Some times they make those install disks check the BIOS to make sure the Model PC it is being installed on is the correct one. But not all the OEM's do that.
To clarify,,, OEM = (Dell, HP, Sony, Toshiba, etc)
System Builder OEM = Joe Shop Owner, Friend who purchased System Builder OS from New Egg, or other of same nature.
The above 2 are locked to the first PC they are activated on and can not be transferred to another system.
Retail = You bought it at Best Buy. This can be transferred to as many systems as you like as long as it is installed and activated on only 1 at a time.
then later use the vista system image thats on a seperate data partition to go back to vista
The answer is maybe. If you mess something up during the install of 7. Then it is possible you will wipe out or possibly cause changes that could affect the ability to use the restore partition on that drive.
IF you are careful, then yes, you should be able to install Windows 7 on the drive as a dual boot OS and later reinstall Vista back to Factory Default using the image from the Restore partition.
IF you are using a System Image created later from within Windows, then you will more than likely blow away the Factory Restore partition.
But there is more to it than just slapping the OS Install disk in and clicking install.
You need to make sure you have enough room on the drive to create another partition to install the second OS. If not, you will need to make the room using a Partition Resizer software.