I've been meaning to find out the truth about all this vitamin C legend.
From what I've read and researched, Vitamin C can intensify the LSD trip. I have never heard (from a scientifically knowledgeable source) that it has an effect on any other drug, except maybe decreasing a methadone high.
It seems that young people using drugs lately think that Vitamin C boosts mushrooms, ecstasy, opiates, other hallucinogens, etc etc etc! Is this just an ignorant 'grape-vine' effect or is there actually something to this besides here-say??
Regarding your question about Airborne; one tablet of Airborne contains 1000mg vitamin C (im pretty sure), which is a decent dose. I would usually pop 3000-4000 mg Vitamin C (buffered to reduce acidity) every few hours while tripping. I think more than that would be pretty extreme. But like I said above, Ive only tried this for LSD. I have no information about what it supposedly does with MDMA, or what dosage would be appropriate.