Since vending machines don't barter with your customers, chances to sell things via vending machines happens independently of customer / Recette interaction. (You can hear a 'kaching' sound whenever a customer makes a purchase, and this can happen in the same instant that another customer walks up to be helped; alternatively, it can trigger when a customer fails the roll to buy an item, in which case you hear the 'kaching' sound but no customer approaches Recette.)
In practice, you will sell more items with one display case converted to a vending machine than you would with just Recette at the counter alone. The principle here is you can sell more items, or you can make more profit per item. Individual preference for these may vary.
This means, among other things, that items with low prices (e.g."Price of Food has fallen") will be bought for their normal base price, selling the item despite the economic downturn for whatever category it's in.