Vyvanse is a prodrug, which means that dosage largely controls duration of effect rather than intensity. I suspect you eat a high protein breakfast or otherwise have a physiology that turns the prodrug into dextroamphetamine more quickly than expected. If Vyvanse doesn't last as long as the manufacturer assumes and comes on more intensely than you need, it's likely a good idea to switch over to dextroamphetamine with an actual extended release mechanism (and don't defeat the mechanism, of course). The combination of the effect being too intense and not lasting long enough is pretty rough to deal with, psychologically.