Sorry to rain on your theory but since i actually work in vaccine mfg for Merck i can say that we do not use multi dose vials for any of our products. The last one was the pneumovax vaccine but we recently stopped, everything is either a 1 dose vial or a 1 dose, pre-filled syringe.
Our company actually makes these vials (although we sell to Merck, you are not one of our larger customers...I assume you are buying from our competitors) and yes it is absolutely true that there has been a constant trend toward more and more single dose (2ml and 3ml) vials. That being said, I believe there are still some vaccines going in to multi dose vials (5ml-10ml) at your competitors, but they are only a small fraction of the totals and are more likely to be specialty items, not high volume stuff like MMR or the flu.