I’ve been on Vyvanse for almost 3 years and that’s one of be things I noticed when I first started taking it too. In my experience I can’t say it’s always the case that it’ll decrease the rate of stuttering because sometimes medication or not my stutter will persist and the rate is unaffected. But there are definitely some days where I feel it does make a difference but ultimately it’s very hit or miss with me. There’ll definitely be other factors that affect this though, like I’m a woman and during the luteal phase of my cycle for example, vyvanse becomes less affective in treating my adhd symptoms for that week every month so potentially that could also be a factor affecting my speech when on the medication. Another is just high stress situations where ultimately if I’m feeling anxious enough any aid the medication may be giving to my fluency is overpowered by anxiety which I know aggravates my stutter causing my fluency to be worse than normal