You also didn't indicate which distro you are using. Fedora Core will install a small boot partition and an LVM partition. If that is the case, you could reduce the size of Vista's partition, and add a partition with the free space to the LVM volume. You may need to resize/move partition(s) on the volume after the new partition is added to the LVM volume.
You may get into less trouble if you reduce the Vista partition using Vista's resize tool. You probably have a small boot partition, a swap partition and a large partition for the rest of the system directories. If that is the case, using gparted (from a live distro) you can move the boot and swap partition (sliding them to the left) and then move & resize the large partition (sliding it to the left and then increasing the size).