PostgeSQL 9.3: Is it safe to stop VACUUM FULL?
Yes, it is safe. Vacuum full will recreate the tables as new objects and only when they're done they will be put to use. If you cancel it, the new files not yet put to use will be removed and old ones will be kept.

Of course if you would cancel it by killing the process handling the vacuum there would be files left, but still the database would be intact.