You've deleted the partition, but you haven't told the system to use it elsewhere.
I'm not familiar with Vista as I refuse to use it, but it may be possible to tell the Disk Management system, to use that extra space. If not, you will need a third party utility, like Partition Magic, or the free Ranish utility to merge the two partitions into one.
Alternatively, you can delete both partitions and then create one big one. But be aware if you do that, you will destroy all the data on the drive.