This worked for me:
- Installed Fedora 18 along side existing Vista, letting it overwrite the MBR with Grub
- Booted to Grub, selected Windows and booted to Vista
- Used BCD to add Fedora 18 as a GRUB2 (not Grub!) boot
- Used BCD to restore the MBR
- Rebooted, and everything is working.