Control Panel > System > Advanced > Startup and Recovery. Does Vista show up in Default operating systems? Probably not, if it doesn't then click Edit. Depending on how many partitions you have this next step might vary, it should say something like
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
where the partition number is different. What you need to do is copy and past the line you have and put it under Operating Systems, so it should look like this
[boot loader]
timeout=30 (this just means how much time you'll have to pick an operating system)
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
Now let's say you have XP installed on partition(1) and Vista on partition(0). You'd want to change the second line to
multi(0)disk(0)rdisk(0)partition(0)\WINDOWS="Microsoft Windows Vista" /noexecute=optin /fastdetect
for it to work. If you do this correctly you'll see these two choices when you boot up. But for god's sake don't go changing Default Operating System before you know what line to use.