What role does Vernet play, in Chapters 42-45 and is it believable?

Vernet's role, in Chapters 42-45 is important, if a bit too convenient. He is the bank president. He helps Sophie and Langdon to get to Jacques' safe deposit box and to escape the building. He even leaves with them in the armored truck. Vernet says that he just happens to be a friend of Jacques' and that he wants to help Sophie and Langdon. Since they have no time to argue, they accept the help. However, just how Vernet knows Jacques and what Vernet's motivations are remain unclear.