How would Executor know Vader was on the way? Easy. I'm sure that somewhere amongst the ion drives and turbolasers and deflector shields on the various ships there are identification transponders. When Vader's personal dropship leaves the command vessel, the vehicle traffic control officer monitors its position and status. When Vader says "bring my shuttle," the pilot signals the VTCO to announce their inbound status and verify a clear docking bay. That, or some very similar standard operating procedure, "alerts" the Star Destroyer to "prepare."
The Imperial fleet is a military organization and as such must operate in a state of continual readiness. Also, remember that Vader is a guy with a documented ability and habit of murdering people, even admirals and at great distances, for disappointing him. Thus there is a very real incentive to make sure that there is a proper docking crew and an officer to deliver any needed updates whenever he arrives.
D_V, the way you worded it, it almost sounds like Vader's in the limo telling his personal assistant to call his publicist and make sure there's a fawning entourage and cheering crowd lining the red carpet when he gets to Burger King.