Vending machines use various methods to detect which coins are inserted, including weight, colour and shape. Light detectors are used to tell the machine about the coins, and the introduction of a new coin would presumably just involve updating the detector's ROM chip - not a big deal.
As for parking meters - in London they are becoming a rarity. Most local authorities are introducing pay by phone parking systems. The meters that do exist are, again, smart meters so they aren't a problem - easy to update them to recognise new coins.