No need to call ahead - they can only tell if the flight is full; not whether VDB is needed.

You should ask at check-in but most likely you will be informed that VDB will only be dealt with at the gate. The check-in agent (or in FRA or MUC the special VDB desk) will write in the system that you are prepared to do VDB and write "VDB" om your BP. Only if the flight is really really over booked your VDB request will be dealt with at check-in/ticket office (happened to me on a recent FRA-SIN flight with 15 pax for VDB needed)

It all happens at the gate. The FM is the key person to speak to. He/she will know if VDB is needed - it is a very good sign if he/she keeps your BP but you may nevertheless find that your seat is not needed at the end of day.

Good luck.