To edit grub boot menu just go to console and type in
gksudo gedit /boot/grub/menu.lst
enter your password, and remove all records corresponding to Vista. To add that option back later, just re-enter those lines. Or backup that menu.lst file before making any changes, like this:
sudo cp /boot/grub/menu.lst /boot/grub/menu.lst.backup
and restore from it later.
You can find more info about boot options in Community Ubuntu documentation, in the Change Boot Options Permanently On An Existing Installation section.
ED: If you just want to remove it for convenience, so that Ubuntu would always boot first, then you would be better off just changing the boot order. To do that start editing menu.lst like was shown previously. Find the line containing default # (where # is some number, 0 by default) and change # to the number of your desired operating system in the boot list. You can also change how long boot menu will be displayed. Just find the line containing timeout # (where # is some number, 10 by default) and change it to the desired time in seconds.