This is the passenger time Table. Purely going by this schedule and given that Ernakulam North (ERN) comes before Ernakulam South (ERS), VB should overtake somewhere in between TCR-ERN. However railways would probably follow the WTT (working time table). Not sure if the WTT for VB is diff at ERN. In that case probably overtake may not happen. There is almost a 1 hr diff by the time both reach TVC. VB reaches at 22:35 and Rajdhani at 23:35 !