((So you'd actually want to throw it over the tank, not at the tank itself, because throwing it at the tank wouldn't kill many people zombies... HOWEVER... I have a plan ))
The propane tank leaves Vincent's arms just as Sammael grabs him. The tank flies through the air, given such a powerful throw. It hits the tank with a resounding CLANG and bounces high in the air, propane gas leaking out of a newly formed hole. The propane tank lands behind the flaming tank and explodes, causing pieces of zombies to rain down upon the surroundings, but by sheer luck, causing the tank to remain in nearly the same place as it was before, blocking the advance of zombies.