When I started taking Adderall I noticed a 5-10 pound drop in weight over the first few weeks. I wouldn't worry too much, for me at least, it seems that after a month or so, your body somewhat stabilizes it's metabolism to compensate for the lowered food intake. In other words, you might loose a few pounds to begin with, but you will reach a sort of "equilibrium" point where the weight loss will stop. Also, as time goes on, you will find it easier and less horrific to eat while on your vyvanse, adderall, dexedrine, whatever.
Over all, tolerance will build up and change many aspects of the drug. Give it anywhere from a few months to a year. Also, the major concern with rapid weight loss is associated with the toxic compounds and ions that build up in fat tissue. Metals and non-polar junk loves to accumulate in fat... Normal exercise and diet allows for a slow, steady weight loss, allowing for your body to break down all that shit. However, if you loose a bunch of weight all at once, your bodily fluids will be rushed with all that shit, causing it to accumulate in other fatty tissues (i.e. your BRAIN).
