I doubt accepting unconfirmed Bitcoin transactions as payment would cost a machine operator much in double spends. It's too much work to attempt a double spend for a bar of chocolate.
double spends are actually easier then you think.
small items are treated as dust. so send a payment with zero fee to the vending machines address. and also on a phone send the same inputs to your own address, but with a fee.. 98% of the time the tx with fee going back to you will get accepted. and the zero tx fee drops off the mempool unconfirmed.
all it takes is 2 transactions.
to make it easier. if you know the PUSHTX addresses of pools, to API call your tx (with fee to yourself) to the pool direct, and know the ip addresses of the vending machines block explorer to send the zero fee tx. you can time it right to send the zero fee tx to the vending machine. and as you hear the machine release the food(seeing tx) send the other tx (to yourself) to the mining pool direct to get the better chance of both "businesses"/nodes not seeing the other tx first
it doesnt require any expensive equipment. just good timing.
on a different note.. im thinking the change over from proper food to the soylent stuff is more to do with not having much "customers", afterall organic foods start to go off quite fast, yet soylent has over a years shelf life.. making it a cheaper product to sell due to less waste items being left unsold