You can shrink Vista, but not extend W7.
Not directly from Windows, that is.
It won't allow a movement of the start position of the partition.
The BCD identifies the OS by a UID which is a combination of the disk signature and the partition start location. Moving the start breaks the boot, so Windows doesn't allow it.
You can do what you want with a bootable 3rd party partition manager, but you'll need to repair the BCD afterwards Recovering the Vista Bootloader from the DVD - NeoSmart Technologies Wiki
unless the partition manager is very new and auto-fixes the BCD for you.