I was just prescribed 50mg of Vyvanse in place of Adderall XR (I'm not sure why I was changed meds? but I think Shire is really pushing this drug on doctors). Anyways I haven't tried it but there are some potential advantages. Vyvanse is an altered version of d-amphetamine (the stronger stereochemical compared to l-amphetamine - higher receptor binding affinity, I'm guessing). Adderall IR and XR are a mixture of both d and l and their salts.
I've seen posts about the stomach and metabolism, but I'll explain how amphetamine goes through the body. The stomach really has nothing to do with the drug - except - an acidic pH can break down amphetamine. This means less of the drug is likely to get absorbed into the bloodstream (this is why you see posts that say you should take tums or mix baking soda with water before you take adderall, dexedrine, etc. and avoid juices). The remaining undestroyed amount is absorbed in the GI track and into the bloodstream. From the bloodstream, amphetamine is broken down in the liver.
Now we need to deal with Vyvanse. The amphetamine NH2 group is protected by the amino acid lysine, so pH will have very little of an effect. All of the posts about mixing with an acid (or lemon juice) aren't correct. Since the NH2 group is protected, the drug will survive the stomach acid much more efficiently. Lysine is cleaved off in the small intestines by the enzyme
trypsin leaving d-amphetamine to be sent through the body. The d-amphetamine hits the DAT (dopamine transporter) and increases extracellular dopamine (and norepinephrine and serotonin) i.e. the euphoria and stimulant effects. After doing it's job, d-amphetamine is sent to the liver for further metabolism. Vyvanse is a pro-drug and must be activated via the enzyme trypsin before it is of any use. Since Vyvanse is "protected", the release is controlled, which isn't good if you are expecting a sudden/sharp effect that Adderall IR or dexedrine offers. Also, Vyvanse can't be abused intravenously or by insufflation. However, one major advantage is the fact that it isn't torn apart by stomach acid. A study shows that 50% more of the d-amphetamine from Vyvanse is present, than if plain d-amphetamine is exposed to the stomach prior to absorption.
My theory and I'm sorry I can't provide quantities or specifics (I haven't tried it and would never promote the process

) is to obtain trypsin from a biochemical supply company and make a solution or purchase a solution. Trypsin can also be bought as a supplement if I'm not mistaken. If the Vyvanse powder is combined, an enzymatic reaction will occur and you are left with d-amphetamine, trypsin, and other binders/fillers from what is in the pill. The extras don't really matter and the amphetamine is now "activated". For example, 25 mg Vyvanse could yield roughly 7 mg of d-amphetamine (equal to 10 mg Dexedrine) This equivalency is not based on personal experimental data and is in the FDA approval guide (link below). However, it should be expected not all of it will be converted. Optimal pH for trypsin to work is around 8.
*I just realized that with this procedure you are left with a liquid. I don't know the specifics on this (melting pt, ect.) I'll look it up in my CRC Handbook and Merck Index, but I'm assuming that you can carefully heat the solution in a watch glass just enough to evaporate the water to form a powder precipitate. Note: If the temperature exceeds the melting point of amphetamine, you will lose a lot of your product.
The reason I believe this will work is because trypsin is added to baby food to predigest it. Another basic experiment shows that when added to milk, trypsin breaks down casein proteins leaving the milk translucent. What is great about this reaction is that it doesn't involve harmful organic chemicals and really isn't a chemical synthesis (just doing what your body does naturally) and any trypsin left in the final product is harmless.
The FDA isn't stupid. There was a real push from Shire to have Vyvanse to be scheduled as III instead of II (There are some executive memos in the FDA link below). I'm sure the FDA took into account all possibilities.
http://www.fda.gov/cder/foi/nda/2007/021977s000_ClinPharmR.pdf
http://www.ncbi.nlm.nih.gov/pubmed/15034119
(I have a B.S. in Biochem so I'm not just pulling this from my a** either)... I love it. It takes Shire Pharmaceuticals close to a decade to make a "very low abuse" stimulant and we can undo that in 10 minutes.