When I first began teaching philosophy, I had a standard comment on assignments for students whose writing was unclear:

While you understand the content, you are having trouble getting down on paper what you know. Note the areas that I marked as unclear in your work. Please see the Writing Center for help with your writing, as it is holding you back.

I assumed that unclear writing was a writing problem, and I softened the blow by distinguishing it from thinking problems. As a result, I was constantly writing “vague” on students’ work and asking them to see the writing tutors.