In the show Futurama the character, Bender frequently uses a coin on a string in vending machines (suicide booths) to work the machine and keep his coin.
Is this possible?

I would think no...maybe in older vending machines, but in today's machines I think that the string would most likely get entwined in the inner workings of the machine.

It doesn't work on modern vending machines. There is a small peice of metal that is pushed out of the way as the coin goes into the slot, and it is designed so that it can only be pushed in the one direction. Once you toss a coin down the slot, you can forget about getting it back unless you have the key to the vending machine, or are willing to break it.

Vending machines have several secuirty features in them.
1. The one way devices that was mentioned.
2. Razor blades to cut any strings that are used.
3. Devices that check for holes in the coin.
4. Devices to check the weight and magnetic strength of a coin.

A coin with a string will have the string cut and the coin rejected.