Download the gparted live cd, burn it to a CD, boot from it, and use that to delete the XP partition. Then you can also use it to resize the Vista partition. This is a Linux Live CD that will boot up and let you change the partition scheme.
Note that if you've got the XP partition in the beginning of the harddrive, which is likely considering you installed XP first, resizing the Vista partition to take up the remaining space will take a while because it will move all the contents of the partition left by however big the XP partition was. Also, NTLDR, the Windows bootloader, will be unable to find the Vista partition and you'll need to use a Vista install DVD to repair it.
You could also use something like Partition Magic to do it in Windows but that costs money and gparted is just as easy to use. As far as I know you would still have to reboot to do the repartitioning anyway, and NTLDR would still lose your boot partition information because it's kind of stupid like that.