It shouldn't be too difficult. You'll have to edit your /boot/grub/menu.lst file if grub doesn't detect Vista automatically.
Let's say that Grub puts a line like the following in your menu.lst:
In my case, I am running Linux Mint (yours would say Ubuntu), and it's located on (hd0,2), which translates to first hard drive, third partition (actually, in my case, only the /boot folder is on that partition, but that's not important). What's important is seeing that grub is booting Linux from hd0. That would mean that if you only have two HD's, Vista would boot from hd1. If Linux is being booted from hd1, then Vista is on hd0. It's hard to tell for sure which one will be detected as which without checking in linux, and checking your menu.lst is one easy way to find out.
title Linux Mint, kernel 2.6.24-19-generic
kernel /boot/vmlinuz-2.6.24-19-generic root=/dev/sda5 ro
So, let's say that your Linux partition is on hd0. Then, in your menu.lst file, find the line that says "### END AUTOMAGIC KERNELS LIST" (I think that's right, I've edited my menu.lst so many times, and it appears that I accidentally deleted that line - don't do that.) After that line, you'll want to add in the following:
...unless you have reason to believe that Vista might not be installed on the 1st partition of the drive, in which case, you might have to figure out which partition it is on and change (hd1,0) to (hd1,1) or (hd1,2), etc...
title Microsoft Windows Vista