darklordoftech said:

Interestingly, the ANH shooting script calls Vader the "right hand of the Emperor" but also calls him Tarkin's "powerful henchman."

Vader's role in the Empire seems to have shifted somewhat in between ANH and ESB. In the early drafts the name Vader actually referred to the Tarkin role (commander of the Death Star and not a sith) and a "Sith Knight" called Valorum played Vader's part, so his role is probably kind of hard to pinpoint in the pre-ESB canon of ANH.
The current canon(s) seems to suggest that he takes orders from Tarkin mostly just because the Death Star is his battlestation, whereas Vader generally has a much bigger role in the Empire.
I personally have always seen Vader as a sort of Shogun figure, which makes sense considering his outfit, especially the helmet, is based of a Shogun's armour. In ESB and ROTJ he seems to be able to command entire fleets at his own whims, and he only answers to the Emperor, e.g. he's a type of Shogun.
However, ANH, and even some EU material seem to suggest that he's more of an "errand boy" for the Emperor, and that although he has the power to do pretty much as he pleases, he has little over-all influence over the Empire's military campaigns.

I'm sure there's some source-book from the 80's or something that explains the whole hierarchy of the Empire. There's COMPNOR of course, but that more bureaucratic than militant.

EDIT: There is an Imperial Sourcebook published in 1989. This should have some pretty detailed info about who's what in the Empire and how it's all run.
The guy who wrote the 1994 edition of AGTOSWU apparently contributed to this book as well. However, it probably doesn't have much more backstory than what we've already got, but it's probably an interesting read.