I assume you want to install on the existing Vista partition? If so,you could extract the.iso onto the HD and run setup.exe.
If you want to install to a separate partition ( which I suggest you do ):
first make another partition and format it
then download the free version of PowerIso, mount the 7 iso, run setup from there and select the partition to install to - during the 7 installation procedure. PowerISO - Create, Edit, Extract, Mount, Compress, Encrypt, Split ISO file, ISO/BIN converter, Virtual Drive, DAA
The free ( unregistered ) version will do what you need.
Just click download on the PowerIso link.