Hi Bluna, Yeah the stimulants can cause fatigue, usually in high doses, and usually after the stimulant phase of effect has subsided. (Think how tired you feel if you drink too much coffee for example; the initial “wired” feeling becomes replaced by fatigue.) Instead of trying higher doses, have you tried going lower?
You’re right that Vyvanse and Adderall work similarly — they are both amphetamine-based stimulants. (http://www.attentiondeficit-info.com/pdf/medication-adhd-quebec.pdf) Either one by itself can cause fatigue, but also the switch from one to the other might also be causing your fatigue.
Keep seeing your doc of course, but one tip that might help: try drinking more water and see if it helps. Sounds trivial, I know, but you might see a difference. Let us know how it goes! 