Vintage 1979 Peugeot roadbike rear derailleur change
If you have friction shifters, they don't really care what kind of rear derailleur you put on them -- just that they can pull enough cable to move the rear derailleur from the highest gear to the lowest gear. 

If you're not concerned with having a matching vintage set, I'd suggest pop  picking up something like a Shimano Altus or Acera rear derailleur. 

But you may want to have the bike checked out by a bike shop -- when a rear derailleur goes, it may bend the mount on the frame (easily straightened with a tool bike shops have) and if it hits the spokes of the rear wheel, it may de-tension or damage some of them (this can be a bit more annoying).