Assuming you can boot Ubuntu, open a terminal (Applications, Accessories, Terminal) and run these commands:
The first command will display the "menuentries" in your Grub2 menu. Count the menuentries starting from 0 and determine the Vista entry (e.g. if it's the third menuentry, the number would be 2).
grep menuentry /boot/grub/grub.cfg
gksu gedit /etc/default/grub
Next edit /etc/default/grub and find the "DEFAULT=0" line and replace it with the new number. Save the file, then run:
If you need clarification it's explained in the "Tasks" link in my signature line.