in mastering a vectorscope tool can tell - among other things - if the sound image of the mix is not too wide. Beyond the 90 degree zone sound is said to be too wide, to be properly reproduced. Is that correct?

If yes, I wonder what to do when the mix is rather narrow, but there are moments that go far beyond. Are excessively parts to be taken as intruding and treated (how?), or can they be ignored?

Beyond that point you might get phase cancellation issues when it's summed back to mono. In stereo then it might sound ultra-wide, but it falls to pieces in mono. Don't be too religious about it. If it sounds good in both stereo and also when you check it in mono, then stop worrying.
Also a good test is to listen to it on a home cinema set. That may interpret your material as Dolby Surround, which works on phase differences. I had a mix once where the vocal reverbs were decoded to the rear channels only
Don't let your tools be the judge of that, rather depend on your ears for that. There can be moments in your track that just don't have that much stereo spread, and later on it's overwhelming. Think of it as building up tension and release, variation which makes it interesting to listen to.

That's pretty much okay, if it sounds good. I often employ this technique myself. Make narrow mix, then in certain sections start increasing width until it seriously goes out of phase, then quickly reduce width. It's great for building tension and releasing. Even if the sound collapses in mono it still usually sounds good as the sound stays out of phase for a short amount of time. I would worry only if sound stays out of phase for a longer period.

Thanks for your answers. So vectorscope is just a tool which can attract attention to certain moments in the mix that could be troublesome, unless that's the way they are actually meant to sound

beatles have stuff far left and right and you know they were quite successful

just put bass freqs in mono (cause they're not quite directional psychoacoustically anyway) and you're set.

The vectorscope tells you how different the left and right channels are. It does not know if those differences are good (makes the music sound wide) or bad (mono incompatibility).

If, for example, you pan a guitar hard left, then double-track it and pan that hard right, you'll end up with very little commonality between the two channels and the vectorscope will show near-maximum width. And it'll sound good.

If, however, you were to clone the first guitar part, flip the phase and hard pan the clone opposite, then the vectorscope will still show the same thing: that L and R are very different. But it will not sound good.

Having parts of the song narrow and other parts wide is a time-honored technique as old as stereo. Watching a vectorscope while listening to your favorite commercial recordings can be quite interesting.