Microsoft changed the MBR when Vista was first released. While Grub sometimes works without problems, I've seen a number of instances where it does cause problems when combined with Vista.
As a result, I normally suggest people who want to multi-boot a Vista machine use EasyBCD. It allows the Windows boot loader to be modified and avoids any chance of upsetting the silly thing.http://neosmart.net/dl.php?id=1
Not sure if they certify it for use with Windows 7, but I've used it with a lot of Vista installs without any problems at all.