Installing Vista works fine without erasing your OS X partition. Here are some instructions I used that worked perfectly.
"1) Boot to the Vista DVD.
2) When I got to the "Choose a drive to install to" page, pressed "advanced" (or "more drive info" or whatever it's called).
3) Selected the first partition (200MB unknown partition), this was the EFI partition.
4) Pressed "delete."
5) Selected the Windows partition to install to.
From there Vista took about an hour to install on my build. After the first reboot it came back with a "Could not find Winload.exe" error.
To fix that, I booted to the DVD again and selected "recovery options" - it said "Your startup information is invalid, click Repair And Restart to fix it".
On the next reboot it worked fine."
Taken from this thread:
http://forum.osx86project.org/index.php?showtopic=14448&st=40
NOTE: The Boot Camp drivers CD will not install properly so the wireless networking won't work out of the box. The regular ethernet driver works straight out of the box with Vista as well as the graphics driver, sound, bluetooth. You need to extract the drivers manually from the setup.exe in the boot camp disk to get the wireless networking driver, brightness control and CDEject.exe or whatever it's called. The startup control panel will also need to be installed manually. At least that is the case for the iMacs.
Note #2: Deleting that EFI partition will make it so you can't erase the windows partition from withing the boot camp app in OS X. You would need to restore the EFI partition manually if you wanted to do that.