He commands a Capital ship. Known as the Executor ,which is a Super star destroyer ,a lot larger then an ISD.
If he is on this ship,he came for a reason... otherwise The Commander would report to him via hololink or in person.
Examples in point.. Empire Strikes back when they are looking for the falcon,which they think is hiding within the asteroid field and they even lose a Star Destroyer whilst talking to him,as on the hololink the guy shows terror and then the hololink disappears...
The Only Two times in the movies he leaves his command ship is to go down to Endor,when he is told Luke has been Captured. or to the second death star when the Emporer has sent him to personally oversee the speed up of it's construction.
This also is a Size Comparision
This is a Size Comparison.
If Vader is here with the Executor as well, we are in a world of hurt.
The following link shows you,the holo communication between ships and the guy dying...
https://www.youtube.com/watch?v=4gh7AcuxfxI