Man I would say both the cars are the same except for the fact that the price and size (looks also are the determining factors. Vitz is bought for its superiority over the local cars like santro,cultus,etc as it has better features,mileage,looks and Power (engine is good). The Platz has a small problem and that is the looks,engine and price. The engine is not that suited for a car that big. When a person gets a Platz the 'CNG' removes the boot space which removes the reason for why they bought the Platz over the Vitz. As I have driven both the cars ... the Vitz has a better result in all its features as its Compact,Agile and has a better power to weight ratio! Looks also are cool.
In Short:
Vitz=Small Car Category,Fully loaded,Excellent Shape and Excellent Engine (not much of a comparison to the Local cars,its better than them for now!)
Platz/Belta=Mid Car Category,Fully Loaded,Nice Shape and Average Engine according to body [1.0L] (Comparison on prices were debatable but corolla and other brands now have given it that) - The main reason of Platz or Belta over Vitz is boot space which is removed after CNG is installed. So overall Vitz wins!!!!