Hmmm.... that's too bad. Must have been a flaw on the dropout forging..... There's a possibility that the dropout can be welded back together but, the break being so close to the brazing on the chainstay, welding heat could damage the brazing, so maybe best to to have the dropout replaced. I'm wondering though if it is easy enough to find the appropriate Vitus(?) dropout to do that.
Definitely worth it to fix that Criterium, but finding the "correct" dropout could be more of the challenge. Maybe you can consider having both replaced by a builder with more common Campy or a Simplex set of dropouts, sourced from dead frames. But are more robust in design and easier to find. Paint damage should not be an 8ssue as the blue paint in the Gitane shouldn't be too hard to match close enough.