I would go for the Vivitar 283 or 285 too- these do great automatic flash exposures without dedicated circuitry. Just right for manual/mechanical cameras. Drawback though is the heft and weight of these flashes. The 283 is almost as big as the IIIf. Add to that the notoriously 'soft' feet that these Vivitars have. But otherwise, I'm a fan of the 283/285. I just bought one two weeks ago, new (the 283 has been discontinued recently), and use it as an all-around flash for my film and digital slrs and Rfs.
Re using flash on IIIf, make sure that the sync dial is properly set to electronic flash settings. For black dial, it should be "2" at 1/30sec; for red dial, it should be "0" at 1/25 sec- though it's supposed to be possible to synch electronic flash with red-dial IIIf at 1/50sec with the flash dial set to "20".
Jay