I am assuming that you know that Vyvanse is an upper and Valium is a downer. Combining these two different types of drugs in high amounts is notorious for causing cardiac arrest, however, I have mixed amphetamines and benzodiazepines multiple times and I am still alive. The key here is not to take a gigantic dose of either, rather, take moderate doses of both.
Vyvanse is a chemical called lisdexamfetamine, I am pretty sure that this drug is metabolized into dextro-amphetamine, and this metabolic process is responsible for the extended release properties of the pharm itself.
IMO, amphetamines somewhat counteract the effects of benzodiazepines. The only reason I could think to purposefully mix them would be if you had low amounts of Valium, and you wanted to get fucked up off them rather than falling asleep (It seems that for me anyways, low doses of benzos will only make me sleepy, whereas high doses turn me into a totally belligerent ass hole)
A note of caution, benzos often times make me forget pretty much everything... They make me forget shit to the point that I have forgotten what drugs, and how much I took. If this is the case for you, mixing the two drugs could be EXTREMELY dangerous. I suggest that perhaps you post some sticky-notes all over your room with friendly reminders like "you took X amount of pills! Don't be a dumb shit and take more!" or "You are on Xanax, don't punch your dad this time!" etc. You get the picture.
Personally, benzos are my least favorite drug, in fact, they are some of the only drugs I would not take given the opportunity. With all these things said, have fun, be safe, and don't over-do it!