For example, it would be acceptable for me to tell a friend "Meet me at my house at 3pm" if said friend knows where my house is. The vague instruction of meeting me at my house would be unacceptable for a friend who knows not where I live, though. I would need to give more specific directions for the latter friend.
In the more frustrating case, there are sometimes vague statements that can be interpreted in numerous ways (making them not only vague, but ambiguous) that are labeled as "generalizations." In general, generalizations are definite line-crossers into the unacceptable vague category (I realize the contradiction of making a generalization about generalizations). A sample sentence: "Anger always lead to hate." That is an amazingly vague generalization that numerous people could interpret to mean a plethora of different things. I won't go into the logic of how that statement is very unsound (let me count the ways). Oh goodness, now my brain is hurting from thinking of all the different ways I could prove that sentence wrong... I'll leave if for another blog. Focus, Hollie!
Anyways, instead of saying "anger always leads to hate," the author of such a statement should clarify the type of anger and hate he is speaking of. The author could say "anger directed toward a specific person over a perceived wrong that is not eventually dealt with through the act of forgiveness can lead to a gradual hardening of the emotional state of a person in the form of hate." While that is still minimally vague and very long-winded, it leaves much less to be misunderstood or mis-deduced. "Anger always leads to hate" may sound more fortune-cookie-ish and wins on the side of brevity, but the vagueness of the statement ruins any benefits to be gained from brevity and wise-person-soundingness (yes, made up word).
Of course, I say all this as a barely disguised rant over people always giving brief, vague statements that can be responded to any which way only to be told that my interpretation of said statement is not what the author meant. If people have a point they are trying to make they should just state it instead of masking it in the brief veils of vagueness. If Johnny is a jerk just say so instead of typing "all men are cads." If Betty hurt your feelings, don't say "People will always let you down." If a friend is disappointed in you, don't say "Expectations only lead to disappointment."