I don't have any general concern about vintage handlebars and stems, but I try to be wary. If I hear a creaking noise coming from the handlebar or stem, I replace stuff until the noise stops.
And there are certain stems that are known to be failure prone. The AVA "death stem" is well known. There's also a 3ttt stem that is known to break:
They are pretty common, and they are nice stems, but I would not use that model of 3ttt stem.