Ok there's the neat rule about a plasma missile possibly getting fused to a vehicle and sticking to it even if it moves. Now here's a question: Suppose someone carefully tosses a plasma grenade on top of a vehicle? Could it just lay on the vehicle and keep burning even if it moved?

I think it is possible, but It all depends on circumstances: what vehicle, how it moves and on what terrain etc, so there is no universal rule . It is "why we need GM" territory, case by case decision.

I agree with Jargal. I think I'd just make a roll for that, modifiers depending on the situation.