Does vitamin C decrease absorption of, or interact with, methylphenidate? If so, how long before/after taking MPH can you eat vitamin C?

No. But there is no health advantage to taking more Vit C than your body can use. If you have even a vaguely normal diet you get no benefit. Except for special situations and crazy diets, Vit C is a WASTE OF MONEY. Don't be a sucker.
It isnt going to. have much of an effect. MPH is pretty well absorbed. But then, so is vitamin C. If concerned 2 hrs between the two should be plenty.