Will splitting a vyvanse (lisdexamfetamine) capsule into two powdered doses impact its efficacy? in particular, will it decrease effect duration?

Do not split: Vyvanse (lisdexamfetamine) is a time released formulation. You should not try to split the dose. If yoiu find the morning dose does not last long enough, speak to the prescribing doctor about raising the dose, whcih should make the effect last longer. Vyvans comes in many doses, so it is likely that adjusting the dose will provide yoiu with the coverage yoiu seek.