Almost never.
You should almost never be vague when writing or speaking for a large audience. Approximately everything posted on the internet is for a large audience. Much of it is much more vague than it should be.
You are almost certainly vague in many instances when you did not mean to be vague. This is a completely normal way to speak if you are speaking to less than fifty people, where everyone you are speaking to has basically the same frame of reference as you do, or when you can assume that people will ask you what you meant if it is not clear to them. This covered almost all conversations for almost all people for all of recorded history until sometime around the year 2010. We have somewhat learned to adjust for this change, but mostly we have not. This causes something like half of all drama.
Let's say that I notice that the president of my city's amateur CourtBall association is a douchebag. I am a huge CourtBall enthusiast, and I would love to join for some competitive CourtBall instead of just playing pickup, but I don't want someone to set my car on fire in the parking lot after a game, as he is known to do. Everyone else involved in managing the city's amateur CourtBall association is a relative or childhood friend of his, they are never going to replace him and they are almost as bad as he is. Very few people in my city are willing to play CourtBall seriously due to this, and people constantly bemoan the death of the sport.
In my normal, day to day, life, I can say a series of things that are unequivocally true:
CourtBall is dead.
CourtBall players are lunatics
Everyone who is not a lunatic gets driven out of CourtBall by the freak ghouls that play CourtBall.
We might as well ban competitive CourtBall, because it only causes grief.
Probably I am not going to run into anyone in my city who has a very different opinion and whose feelings I am upset about hurting. On the off chance they play pickup CourtBall and have never heard of the competition scene, they will ask me what the hell I am talking about and I will say "oh, the local CourtBall association is a nightmare". They will get over it. This will take about three seconds of my time. (If they're in the association, fuck 'em, they need to hear it, I'm insulting them on purpose.)
If I post this on the internet, literally everyone on Earth who plays CourtBall, whether amateur or professional, competitive or non-competitive, in my city or out of my city, can hear my opinion. They have no idea what I am talking about and I sound like I am crazy, stupid, malicious, or some combination thereof. I may, eventually, get the chance to clarify that I am referring to my 1) local 2) amateur 3) competitive CourtBall scene, or really to 4) one specific guy and his friends in that scene. Almost nobody will see me saying this because people saying sane things aren't good clickbait. Also, there's a ton of crazy people on the internet, and people just straight up lie about this sort of thing, so there's no reason for anyone to believe me.
(I was going to use pickleball and for all I know my city has major pickleball drama. Plenty of people know what city I live in, and I have absolutely no idea what's going on with any sport I don't pay attention to.)
So I shouldn't be vague. I should keep my mouth shut or I should be very specific about who I am insulting and what I am insulting them for, because otherwise a ton of people I do not intend to insult will justifiably feel insulted since the thing I have said literally and directly insults them.
Just so we're clear: this generalizes to basically anything. It has nothing to do with sports in my specific city. It applies to every -ism, all the -ists, every hobby, every interest, every site, every fandom, and any other social niche. Given that this starts absurd fights about relatively minor things, it seems like it's also a good idea to avoid doing this for any identifiable group of people larger than a specific family.
This runs afoul of norms where some people, sometimes, consider it okay to insult large groups of people if the groups of people are not characterized by any sort of voluntary association. Insulting groups of people based on involuntary traits, that is, things they cannot change has a rich and layered history and my considered opinion is that you should not do it. Anyone promoting a norm around it that tries to be cute and say sometimes you can just insult large groups of people for immutable traits is someone you should hit with sticks.
The Exact Prescription Here
Here are the types of vagueness to avoid:
Who you are talking about: ideally grievances should be with a specific person
Why you are talking about them: ideally you should state exactly what they did
What, if anything, you think should be done. "I have no idea" is a valid answer to this question, and is less ambiguous than not addressing the question.
If you simply do this you can avoid the vast majority of friendly fire on the internet.
Shooting the Moon
You can sometimes say incredibly vague things and instead of everyone feeling insulted, nobody does. This is because they always imagine that you are insulting someone who is not them, and they will all agree with it or at least pass over it. If you are trying to accomplish this, there are a few tricks.
Be extremely vague, so vague that whatever you're saying is a Rorschach test.
Say something that sounds sort of specific, but refers to almost nobody, and then in the same breath imply a much larger group of people, but never say it. You can then act like people are being irrational when they react to the thing you implied.
Make sure you are extremely well-liked before you start vagueing in this way.
Personally I would prefer it if you didn't do this, because this sort of passive-aggressive bullshit is a huge part of why many people assume the worst of anyone saying anything vague. I would prefer it if we could all, collectively, agree to stop peeing in the pool. Be direct when you intend hostility.
I would be negligent if I did not acknowledge that it sometimes happens. Also, I am trying to do it here. Who am I insulting? You have no idea. It sounds sort of hostile but you are pretty sure I'm not insulting you. If you imagine I am insulting someone specific, it's probably someone you don't like. Choose-your-own-adventure. If it matters: I am not actually insulting anyone in particular. Or, I am insulting a few dozen people, one of whom is myself.
Honest.