I believe Vista can only install on a partition formatted as NTFS, so you'll need to make the partition bigger than 32 GB if you want to be able to install Vista straight onto it. Otherwise, Boot Camp formats it as FAT32, which Vista doesn't like...
I just finished doing this over the weekend with RC2, one thing to look out for though is that about halfway through the install, on one of it's various restarts, I had a problem that it came up with a screen that said something about incompatible or failed hardware, and press Enter to continue or Esc. to quit. Well, seeing as there was no power being supplied through the USB port for some reason at that point, I had no choice but to shut it down manually and restart. When it restarts, it tries to boot off of the DVD again, and will seem like it's starting the install all over again. To avoid this, when you restart it, hold down option (or hold down the Menu button if you have your FrontRow remote handy, I find this is actually a more reliable option, given the anamoly of it not supplying power to the keyboard for some reason) and select the Windows partition. It will then complete the install. You may not have this problem, but then again you may, so I figured I might as well give you a heads up. This was on an iMac Core 2 Duo, so there might be some sort of connection...
Hope this helps!