It has to do nothing with BJU and rakes it has. Vaishali's arrival at NDLS is 6.40am & dep from NDLS is 7.50pm, duration of interval 13 hrs 10 mins.
And it's arrival at BJU is at 5.15pm & next dep at 9.30 am, interval of 16 hrs 15 min. Hence even if this train is delayed by 7-8 hrs it departs ON TIME from respective Jn.