I think there was two types of questions on the COMLEX that were "vague." The first is that (especially for micro) they kind of expect you to understand what the "most common" is, and to default to that. For example, if they give you a vague micro questions about a patient with COPD who got pneumonia, you should "default" answer H. flu and then only change that answer depending on the small amount of info they provide in the question stem. I think COMLEX does this type of thing more than the USMLE did.
The second type of "vague" question is just poor wording. Where you can know the concept/answer "fine" but the wording of the question can make you second guess yourself/what the question is asking. Obviously I can't provide real examples from my exam, but here is a pretend example: "patient comes in with symptom X, symptom Y, symptom Z, and labs show ABC. Patient has QRS on physical examination. What is the most likely explanation for the presenting symptom?" You could interpret that in any number of ways. The symptoms that made them come in in the first place? The symptoms seen on physical exam? The inciting event? Things like that. When I would come accross one of those questions on my exam I wrote a feedback note saying "Which symptoms?" as a passive aggressive way of telling them to review their damn questions before submitting them onto our exams.
