This great new tutorial from Barefootkid will show you with pictures exactly how to accomplish this using the best partitioning tool for Windows 7:
Partition Wizard : Use the Bootable CD
Use Option One in the tutorial and substitute your Vista partition for the 100mb boot partition he refers to there.
Read the Note as once you delete Vista partition then Resize Windows 7 into its disk space and mark Windows 7 active, you'll need to
run Startup Repair up to 3 separate times with reboots to write the System MBR to Windows 7 and start it up.
Back up your files and a system image externally as Resizing operations can fail although we have not had one fail in a couple hundred of these operations we have helped with here.