Poor Vernet thinks he's doing the right thing and protecting Sauniére's assets.
He heard on the radio that Sophie and Langdon are also implicated in the deaths of the three other sénéchaux. (Ah ha. Langdon realizes that if they're dead, that's why Sauniére gave the key to Sophie.)
Vernet insists that whatever the box contains, he has no intentions of seeing it become police evidence, so they need to hand it over STAT.
He fires the gun into the truck over Langdon's head to make his point about how serious he is.
Langdon, under duress, places the box on the edge of the truck's cargo hold.
As he does, he notices the spent shell from Vernet's wild shot. He cleverly brushes it into the door's lower sill, and then backs up against the back wall as instructed.
Vernet is kinda flummoxed, because the box is too heavy to lift with one hand (while the other one is occupied with aiming his gun at his prisoners).
So he puts the gun on the bumper, briefly, to lower the box to the ground. Then he quickly grabs the gun again and goes to shut the door, but it won't close properly because of the spent shell that Langdon placed.
As Vernet is still working this out, Langdon crashes the door outwards, smashing Vernet's face and knocking him to the ground.
This allows Sophie and Langdon to retrieve the box, and hop into the truck to make their escape.