If all else fails, just replace the whole thing with the stick from a standard Tandy Color Computer joystick. It won't look original, but it looks good, gives you better leverage (IMO), and is basically a perfect drop-in replacement since the Tandy and Vectrex both used the same gimble mechanism in their controllers. The only hitch is you might not be able to stow the controller in the face of your Vectrex anymore since the stick is longer.
And the Tandy joysticks kind of suck anyway, so sacrificing one to fix a Vectrex controller is a noble use for one.
This is what I ended up doing.
The diameter of the shaft turned out to be pretty tricky. I managed to stretch out one size of brass tube enough to fit over the stub, but then discovered that the shaft was actually bent, so it didn't really matter, the shaft was going to have to be replaced. I was able to find a pair of the Tandy sticks on Ebay for cheap. The stick mechanisms appear to be identical, with the exception that the Tandy uses a longer shaft (on the handle side. internally, it appears to be the same), and doesn't have springs in the pots.
Luckily, the original Vectrex pots are still in good shape. It really would have bothered me if the new stick didn't self-center, so that was a bit of good luck. I simply disassembled the stick mechanism and swapped out the shafts, and I'm done!
And yes, you were right, the controller no longer fits in the face of the console, but that's ok. This one is the primary controller for my secondary Vectrex, and a second/backup for my main Vectrex, so I'm ok sacrificing the ability to stow it neatly in favor of making it usable!
THANKS for the suggestion!
(And yes, I know, it's still in need of a good (better) cleaning, and I'll probably put a new label on it as well. I think it's seen some pretty heavy abuse, but it's safe now.