I think that in ANH, Vader is definitely not anywhere near the most powerful man in the Emperors command. He is more like, as Leia suggests, Tarkin's lap dog. The other officers neither respect or fear him, it's more like they're jealos of him or think he shouldn't be in his current position. That's the feeling I get anyway. However, after the Death Star is destroyed, most of the very senior officers would have been killed, so Vader moves into a position of much greater power, IE one level down from the Emperor. My feeling is that he probably would have acheived this position anyway, due to his "friendship" with Palpatine, but it would have taken longer if this had not happened.
I think that in ANH, Vader is definitely not anywhere near the most powerful man in the Emperors command. He is more like, as Leia suggests, Tarkin's lap dog. The other officers neither respect or fear him, it's more like they're jealos of him or think he shouldn't be in his current position. That's the feeling I get anyway. However, after the Death Star is destroyed, most of the very senior officers would have been killed, so Vader moves into a position of much greater power, IE one level down from the Emperor. My feeling is that he probably would have acheived this position anyway, due to his "friendship" with Palpatine, but it would have taken longer if this had not happened.