Guess you haven't setup your menu.lst correctly.
Go to a Shell and do the following: (if you dont know how: alt+f2 and type gnome-terminal)
su -c 'gedit /boot/grub/menu.lst'
It asks for your administrator password
And then you delete the line saying hiddenmenu so when you boot you can actually see the Menu to select the OS w/o pressing a key. Now you add an Entry like:
rootnoverify (hd 0,0)
where (hd0,0) is the disk where Vista is installed (hd0,0) would be the first partition on the first disk.
Just guessing thats your problem, if not I'm running the same config so just ask.