It's not exactly an extended release version of dextroamphetamine, it's lisdexamphetamine, which is a prodrug for Dextroamphetamine but it kind of does act like an extended release version because it has to be metabolized into amphetamine creating a longer duration. It's also about 1/2 the strength of d-amp.
I don't have much experience with Vyvanse, but I do with pharmaceutical amphetamines and methamphetamine, and what I always did with my adderall once I started to notice I was gaining a tolerance, was cut my dose down by a quarter. The quarter dose reduction wouldn't be noticeable enough for me to feel like I was just completely wasting the amphetamine, it would pretty much feel like my normal dose and since that normal dose wasn't doing much for me, why not cut it down by 1/4 so that in a few days I could take the normal 30mg and feel much more spun.
Also, I don't know if it would work with Vyvanse (although I don't see why it wouldn't), but taking antacids can help the absorption. But really try reducing your dose for a little while to help reset your tolerance. It's way better then to keep upping your dose (which is kind of pointless with vyvanse anyway) and then running out of pills.