The soda vending machines I've seen opened has a 4" by 1" lock type thing with a round keyhole kinda like the `keys' that computers often come with (but bigger).

It works by unlocking it with the key, which makes the lock pop out a little. Then you turn the lock counter-clockwise until it's far enough out that the door (entire front of the machine) opens, at which point you just pull it open.

Remember that it's designed for public places and the inherent abuse this leads to, so you won't get too far with violence.