You will need a coordinator - the center of the mesh. All other devices send data to the coordinator. It then sends data to all the other devices.
With XBee ZB modules, the coordinator need not be the center of the mesh in any particular sense, and also it need not be a data concentrator. But it is needed to initially start the network and allow other modules to join. However, routers also can allow other modules to join, so once one or more routers have joined, this is not the exclusive domain of the coordinator. In fact, the coordinator can even be removed at this point.
If using XBee ZB modules, indeed each network needs exactly one coordinator. I assume the population of vehicles is dynamic. Vehicles will come and go, join and leave the network, so this begs the question which vehicle has the coordinator. Even if this only affects initial network formation, at some point we must expect all vehicles to leave the network, so the question then is how does the network get re-established?
I haven't used Digi's proprietary DigiMesh modules, but I believe the concept of the coordinator node is absent with that protocol.
But I have my doubts, even disregarding the coordinator concerns. I just do not think that XBee modules are meant to move around frequently. Route discovery can cause excessive overhead on the network. As vehicles move around, there will be frequent changes in the routing tables that are maintained by every node in the mesh. Each node will be kept busy trying to determine its nearest neighbors, link quality, etc.
Broadcast transmissions might seem to make sense in a scenario like this but in fact they are to be used sparingly even in stationary networks due to the network overhead they cause.
A system like this needs some fairly impressive intelligence, plus fast and reliable RF networking. I know a little about XBee modules, but I sure wouldn't be choosing hardware until I had a very good handle on the requirements. I'd be doing my homework and a huge amount of research. A fair amount of prior art exists in this area, including some impressive demonstrations, so that is obviously one tree to bark up.