A few years late to the discussion, but I am noticing the same issue with my blue vervain. All of my blue vervain including 3rd year, 2nd year, and 1st year plants are exhibiting the same phenotypes as OP. None of these plants have ever been near herbicide. There is a lot of organic matter and fungi in the soils, but all my other wildflowers are happy. I'm inclined to think its some sort of insect pressure.
