you are all thinking first generation bitcoin transactions based purely on the blockchain. I still think a reputable vending machine will add a loyalty/membership card system where you deposit bitcoins to your membership balance and then the machines deduct the amounts from the membership balance. off the blockchain.
That way coins are preconfirmed when you have deposited your funds into your balance and you are spending the coins much like MTGOX/BTC-e using databases not blockchains for internal transactions.
At my work we are given a Kenco card which has a 2 free coffee's per day and then any other coffee's add to a balance which you pay off on payday.
I would certainly not use a vending machine where I need a special card. Vending machines are something I use in some cases when I want the snack the fastest and easiest way. Having to add credit to a card negates both qualities.