Mix something like sugar or gelatine with the petrol and it will stick like baby poo to a blanket.
Nearly all AFV's are fitted with some form of fire fighting capability within the engine/transmission compartments and a lot of newer ones have seperate system for the crew compartments. In theory a vehicle could carry on even when on fire but theory never works in practise.
Back to PR, I think a realistic option would be severely obscured vision for the driver and commander to simulate the sighting system being obscured by smoke and flames for a period of 1 minute or so. It gives them chance to bail out or move the vehicle to a safe area whilst it's burning. An engineer would be needed for minor repairs but the vehicle should be pretty much functional after the fire is out.
This simulates the vehicle being damaged and temporarily removes the asset from the game but also give the enemy ample opportunity to destroy it if the crew leaves the vehicle in a bad position.
with the right texture the fire itself will obscure vision and that will also act to make sure you get the molotov in the right place.