I am planning to travel to Gare de Vincennes via RER B from CDG to Chatelet-Les Halles and then transfer to RER A. If I buy a ticket at CDG for the RER B will I have to buy a second ticket at Chatelet-Les Halles for the RER A journey to Gare de Vincennes?
At the CDG train ticket office you can ask for a RER ticket that will take you the entire journey from CDG to Gare de Vincennes. They will create & sell you a custom ticket for this route.
You won't be able to buy this ticket from machines (without buying multiple tickets).