Ross Carlson explains how to install Vista on a MacBook Air using Imagex:

1) Create a partition for Windows using Bootcamp (I created a 55GB one since I’m going to use Windows 90% of the time)
2) Reboot from the standard Vista install DVD. Format the partition you created and complete a normal Vista install. You’ll need to do this so the boot sector is written properly. I’ve tried stopping right after you format it, and have even killed it right after the file copy starts (neither worked) so I’m guessing the boot loader gets written at the end of the process.
3) When you reboot be sure to press and hold the mouse button after the Mac bootup “ding” sound. When the disk ejects quickly put in your WinPE DVD with imagex.exe and the image you’d like to install. You may have to boot back into OS X first. If so restart the Bootcamp assistant and choose install Windows.
4) Pay close attention as it boots so you can “press any key” to get WinPE going.
5) Once you boot into WinPE you’ll need to reformat the disk (which doesn’t mess with the boot sector). So run the following commands:

diskpart
select disk 0
select partition 3
format fs=ntfs quick
assign letter=c
quit

6) Now you’re ready to put the image on the machine. For this you’ll run:

imagex /apply IMAGENAME.wim 1 c:

7) Now go get a cup of coffee (or several) while it puts the image in place. When that’s done reboot and go get dinner (and then maybe breakfast) as Vista starts up the installer for the image.