My point is that ... changing how all vending machines work is an expensive proposition. It won't happen )at least existing units in the field won't get retrofitted) unless there is there is a sufficiently disrupting event. Changing the coinage is such an event - the old machines won't work with the new coins - period. But once the machines _must_ be changed, they could add both the new coinage and bitcoin. * At this point, the incremental cost of also adding bitcoin would not be that great.
The most likely event are dropping a coin or a change in the coin's weight. I can't imagine either of those requiring much more than a software update. The addition of bitcoin would require changes in machine design to display the QR code and an actual live network connection so that it can get at least one confirmation before vending the product. I think the network connection will be the biggest hold up for existing vending machines.