Just to preface this, vowels have not been my consistently most difficult sounds to say. However, I do block on vowels once in a while. I just started working on laryngeal blocks with my speech therapist. We've only touched on it in two sessions, but we're working on abdominal breathing, breath control and recognizing the incredible amount of tension that happens throughout my speaking system when I block on vowels (like Andrew - nothing would come out at all!). It is definitely more difficult for me to feel and work through compared to my 'T's and 'D's, but I can certainly say being able to recognize and reduce the tension has been helpful on reducing/eliminating vowel blocks for me so far.