I do not recommend re-dosing (Taking more before you come down from the initial dose) because it is a waste and will just make you stay awake longer, not any higher. If you want to get high on amphetamines, I just recommend you don't start. It can lead to addiction. However, having said that Vyvanse is special and different from Adderall in quite an intelligent way actually. Adderall is Dextramphetamine, while Vyvanse is Lis-dextramphetamine. It may seem like a small difference but it changes the way you can take it entirely. The "lis-" part is a protein (Lysine) that was added on so that the pill must be swallowed to allow the liver to break off the protein (essentially, it doesn't exactly work like that) but then it becomes Dextramphetamine. That way you cannot insufflate, inject, smoke, or do anything other than take it orally. This helps prevent abuse. But I would recommend staying around 80mg or lower because I, personally, am 160Lbs and 80mg does the trick. It is "safe" (You'll live but damage may occur) to take more, but I don't know the threshold or what the dangers are so you may want to research safe doses for your weight and age.
That being said, even just taking 80mg, waiting til it kicks in, and smoking a nice indica heavy medical marijuana strain. It feels very relaxing and the Vyvanse stops the heavy sleep inducing effects from the indica so you just feel really damn high but pretty awake, If you smoke the right strain toward the end of your trip it will help with the come down. Vyvanse makes me, and many others that I know quite agitated and aggresive on the come down and it helps relax you so that you aren't as much of an asshole and you feel nice and relaxed rather than angry.
If you have trouble sleeping, I recommend either smoking more Marijuana to basically make yourself passout, or use Melatonin or other over the counter sleep aides. But again with those, (speaking of the other sleep aides, Melatonin is safe in doses as much as 50-80mg but can result in vivid or bad nightmares from high doses since a normal dose is 3mg-5mg.) be careful because in high doses they are dangerous and can be addictive as well.
Sorry for the long post and lots of info, I am currently on 80mg Vyvanse myself, and high. Unfortunately I couldn't get medical quality, but its still decent if I smoke enough.
----LoGiiK