Gparted is not able to successfully repartition (or resize) partition with Vista or Windows 7. It is a known issue. The only software that is capable of this is Acronis Disk Manager, starting with certain release.
If GParted is used, it breaks Vista and needs to be repaired.
Another way to resize Vista partition is from within Vista to shrink partition, but it allows only some shrinking, depending on size, it insists on leaving lots of unused space.