Also, you need to see unconfirmed transactions to know if coins are heading your way. Otherwise, there will be a minimum 10 minute delay before you know a transaction has been initiated. How would you do a vending machine with a random delay around ten minutes?
You don't do a vending machine with bitcoin, period. If you can't wait ~ 1 hr for 6 block confirmations, then you are at extreme risk of being scammed, and vendors will not rely on promises (i.e. uncomfirmed transactions) to deliver goods. Nobody is going to use a vending machine that you have for wait 1 hr to prove you've paid for your chocolate bar, ergo, there will be no bitcoin vending machines.