If my understanding is correct, lisdexamfetamine by weight is equivalent to about 40% dexedrine chemically, so your 50 mg Vyvanse is kind of like a 20 mg dose of Dexedrine spread across 12 hours, but with much better effectiveness because of the way Vyvanse gets metabolized into d-amp. At least that's my conclusion from reading a lot of the publicly available information on the internet (wikipedia, PI sheets, drug info databases, etc.)
Do you remember how much dexedrine you first started with, the dosing frequency, and how effective the early doses were? I would think that 50 mg Vyvanse would be no less effective than your 40 mg dexedrine if you took all of the dex at same time, but the Vyvanse could be weaker if you split the dexedrine in to 2 or 4 daily doses. An experienced clinician who has prescribed both dexedrine and Vyvanse to the same patients would probably have a good idea.
If you want to learn more and come to your own conclusion, you can research the drugs online. Wikipedia is a good place to start for background and high-level info but is not totally reliable.
For medications of interest I like to look at more authoritative or official sources, such as drug info databases, the prescribing information/medication from the manufacturer(s), etc. (I think the FDA hosts these on their site.) Some drugs (amphetamines in particular) may have scientific discussions on forums like Bluelight.
