From what I read it doesn't seem like a good idea to take psyllium husk together with other medications. They seem to recommend one hour or two of interval between it and other meds. I don't know if it's just doctors being cautious, though.

I usually take Vyvanse just by itself at around 7AM just before walking to the gym, so I guess I associate the effect kicking in to my walking back home. On the days I ate something together with it, or specially with Psyllium, I notice a diminished effect