Not like in Europe, you can find beverage vending machines on streets of anywhere in Japan. If you are in the populated area, may be you can them find each 50 meters or less. Then, each vending machine mostly offer products of the one of the beverage companies only. For example, you can not choose Coke and Pepsi at the same vending machine.
Then, the beverage sold at the vending machines cannot be found easily at the supermarket. It means than the beverage sold at the vending machines and at the shops can be totally different. Usually the beverage sold at the vending machines are more offers than at the shops.
Jeleets Crush Pudding from Pokka Sapporo company is the one you should try from the vending machine. It is a mixture of dessert and beverage.