Vat is a form of consumption tax and multi stage tax ,it is to be assessed on value additions to the product. In the Vat , the user pays tax in the cost of product less any of the costs of material used in the product that have already been tax paid. Actual tax burden is visible at each stage from production to sale. And also tax on inputs can be claimed back from the Tax Department.
As regards Sales Tax, it is to be collected at one stage of sale of goods.
Value Addition Tax is a true revenue to the Government.