Thor's hammer doesn't contain an infinity stone, because we've already accounted for all of the infinity stones. The reason The Vision was able to lift Thor's hammer is because Vision is not a living entity. It's a thing; an artificial lifeform. It's the same reason Thor could place his hammer on an elevator and the elevator would go up and down. If he placed a human underneath the hammer, they wouldn't be able to move, yet the elevator would continue to move from floor to floor. The Vision is merely an exceptionally intelligent and well armed elevator.