Why do I need to use multi drop bus protocol (MDB) in vending machine and How?
If you are building a vending machine, unless there is legislation or client preferences about technology, then build it to suit the market you are aiming for and build it to maximize your profits. It's as simple as that.

On the other hand if you mean you are connecting various off-the-shelf parts together that communicate in a certain way then you'll need to find how that works.