I think the op was talking about vendors and not vending machines.
Good luck keeping track of thousands of indiviuals cups. Why can't the drinker keep track of his own cup.
This article is a few years old. They tried the same thing at Western. A university here in Ont.
Also Tim Hortons, Canada'a largest Franchise, whose sales surpass that of McDonalds, offers an incentive for reusing your own cup. Its the bi annual " roll up the rim" to win a car contest that throws off their own efforts to reduce, reuse, recycle