It can take overnight, particularly for a large Library, a first Vault, or with a slow external drive. I don't know why you couldn't do anything else (that seems odd). What is the format of the new external drive? How is it connected?
The Progress Bar is not reliable. Neither is the "Not responding" report from the Force-quit dialog or from Activity Monitor. (The program might not be responding, but is almost never hung.)
You should avoid force-quitting Aperture (or any DBMS).
You should run "Database Repair" after every force-quit. Instructions are on Apple's Aperture trouble-shooting page.
So back-up your Library (make a copy), run the repair, then try the Vault again, this time let it run overnight with your machine powered on and set to not sleep.