You need to select the Shrink (or Resize - I don't remember which) option, and then specify how much you want to shrink the partition. If you have any trouble resizing (it wont work if you're trying to resize the partition the OS you're running is on), then you can boot from the Ubuntu LiveCD (installation disk...whatever you wanna call it), get into the Live session (by choosing the Try Ubuntu With No Change To My Computer or some such option), go to System>Administration>Partition-something-or-other and that will be Gparted. You can then use that tool to resize the partition from off the CD (so you wont be running of the partition you're trying to resize), and get the space you need. Then once you got the free space, just create an NTFS partition, and install Vista to it.
Cheers, and good luck.
Jake