Sandwiches -- especially anything containing meat -- need fresh ingredients to be actually compelling. Seems like it'd be difficult to beat restaurants, especially with the "yuck/cheap factor" that comes with vending machines.

Plus, going out for lunch is 25% about food and 75% about getting out of the office and walking around in the world.

I'd imagine the primary market for such a vending machine would be the small population of workaholics who are too far gone to take a 30 minute break in the middle of the day, or McDonalds competitors in high-minimum-wage areas. But honestly, it seems like McDonalds would be able to beat any comer (maybe even without having to license any IP).

Tough market :-)