I don't think you can do that. I know with nLite and vLite you need a 64-bit OS to make 64-bit boot disks. However, you could try this option before going any further:
1) Boot the Vista disk and go to the repair menu. Choose the Command Prompt.
If your bootloader is screwed this should fix it. It will repair the MBR and then delete and rebuild the BCD (which is pretty obvious from the commands actually