You can delete and reuse the Ububtu partition from inside the Vista Disk Manager.
Just be careful, since you cannot then reboot without first fixing the Bootloader.
There are two ways to restore the Vista bootloader :
Recovering the Vista or Windows 7 Bootloader from the DVD
Recovering the Vista Bootloader with EasyBCD
And mind your backups before doing any partition work.
Note: The order of things depends on which partition is first.
If Ubuntu is after Windows, then do EasyBCD first and verify that the boot works, then delete/reuse Ubuntu.
If Ubuntu is before Windows, you cannot delete it, since Windows refers to its own partition by its number. Complete removal of the Ubuntu partition will then require the re-installation of Windows. If this is unacceptable, I suggest to do EasyBCD first and verify that the boot works, and only then either reformat the Ubuntu partition as another Windows disk or resize Ubuntu to a small size and leave it alone, maybe moving the Windows partition. This will require a good partition tool available as boot CD.