You do not need to research any modules in Premium vehicles.
Premium vehicles earn more credits per battle and their repair costs are cheaper than standard vehicles.
Experience earned in Premium vehicles can be converted to Free Experience after the first battle and used for researching other modules and vehicles.
Premium vehicles allow for the accelerated crew training from the first battle; the crew receives more experience after each battle. Moreover, you can move crew from any vehicle of the same type and nation to Premium vehicles without penalties and necessity to retrain your crew.
Premium vehicles earn more experience per battle.