This is how I would do it
Plug in the Vista drive as a 2nd device, just like "strollin" said...
as long as you don't change the boot order in the BIOS.
win 7 should boot w/o a problem.
Once you are on the Win 7 desktop, I would proceed to Re-format the Vista drive.
+1 on NOT trying to save the Applications, just make sure you backup any Documents, Pictures, Saved Games so you don't loose those and they can now be on your new Win 7 drive.
Once the drive is formatted, you can now use it as a back up or whatever, even though, with a 1TB drive, I just can't see why you would bother having another drive in the PC but hey... to each his own, right?