I have the same problem: vowels. my second language and most spoken language for me is German, which is kinda known for hard pronounced letters and words. I'm still searching for the best trick but one of the best but also one of the most time consuming and hardest tip is to build self acceptance and confidence. you can try any trick, when you fear your stutter, your stutter controls you more than you control yourself. hard to describe it. try to see it as a friends by the end. it's hard when your stutter is your biggest enemy but hey, having confidence helps you in many aspects in life!