The trouble is that (2) is a boot partition and therefore can only be formatted by reinstalling Windows -- which would hose your current multi-boot.
A possible workaround might be to edit your boot options so that the 32-bit option no longer appears, then reboot and go to whichever 64-bit -- then instead of formatting, just delete the contents -- making sure you don't delete the necessary boot files and folders that are located in (2). MAKE SURE you do not delete the boot files, because doing so would make your computer unbootable.
Just delete the folders (NOT the other files) except for the "Boot" folder
, which is hidden btw -- and leave all other root content on the drive intact
This should ensure that you would be able to boot, and yet keep your current configuration intact, and yet use (2) for data storage instead of 32-bit Vista...