I have 3 Vitus 979s and 2 of them ( early 1980s bikes)have been ridden many times over pretty rough terrain in France when taking part in various retro events eg Anjou Velo Vintage.No sign of any joint failures.[Speed wobble is something different,I think- what exactly happened?]However, these are old bikes and something could go on them anytime, as it could at any time on much younger bikes.These were quality bikes and the fact that they are still around and rideable says a lot.Your bike is quite a bit younger than mine and you’ve had it from new so much less of a worry!
Re stepped ferrule for rear brake cable I think that the one from Velobitz may work ( UK company)
Welcome to Retrobike!