1. Insert and boot to your Vista disk.
2. Under install now, you should see "Repair my computer"
3. Select startup repair.
4. This will break the Windows 7 boot.
5. After this has completed, boot to Vista.
6. Once Vista has booted shut it down.
7. Now insert and boot to your Windows 7 DVD.
8. Do startup repair as listed above.
9. No see if you can successfully boot to both OS.
Also, there are third party applications like the one mentioned above that will possibly make this process easier. If you would like to use one of those programs feel free. If you have questions about how they operate you would need to contact the manufacturer.