You'll run right into problems attempting to shrink the Vista primary any further especially if booted in the Vista installation when trying to start off. First you may run into the lack of memory error when booting into the Vista installation later simply from possibly not having enough free drive space if you should add anything further as far as programs, files.
The second would be the obvious recommendation to back everything up from the drive since the resizing if that should go bad will effect the entire drive. If you manage to shrink the Vista C primary while booted in 7 to use the Disk Management tool there you will still need to move the 7 primary forward afterwards where that will not only take some time but usually need the use of a 3rd party drive tool to start with.
While the 17+gb should still leave enough drive space for Vista a 3rd party tool for seeing this done would allow one operation to follow the other instead of booting into one to do something with the other. If you need a free drive tool for this GParted live for cd can shrink the Vista primary, move the 7 primary forward to fill in the then empty drive space(gap between primaries), and later expand the 7 primary to fill in the space it was moved back from at the rear of the drive.
GParted information and instructions, GPARTED DOCUMENTATION - GENERALITIES
Latest release in iso disk image form, Browse GParted Files on SourceForge.net
Some additional screenshots can be looked over at GParted -- Screenshots
For multibooting on multiple drives and resizing, moving partitions GParted has been quite reliable here.