Gorgonzola's summary of the Demon Princes novels is accurate enough as far as it goes, but I'm afraid he's missed the grandeur and sweep of the forest by obsessing about the trees.
While the pentalogy is superficially a tale of revenge exacted by the increasingly powerful Kirth Gersen on the even more powerful criminal masterminds of the Beyond, the real meat of Vance's series (as it is in so many of his other tales) is the endless, often baroque variety of cultures and peoples that make up the Oikumene. All human, but all different in social structures, appearance, architecture, foods and religion, the inhabitants of the Oikumene reveal themselves to the fascinated reader as they progress with Gersen from planet to planet in search of his quarry.
Here, the secretive and ever-wary poisoners of Sarkovy; there, the inhabitants of long-lost Thamber, living out the bloody medieval fantasies of Kokor Hekkus. Off the main paths of commerce, the austere and lonely Smade's Tavern (on Smade's Planet); in the center of humanity, Old Earth, but not as we know it. Indeed, humanity in the Oikumene would seem to have only three common features: the basic human genetic material, the currency of the Standard Value Unit, and the secretive Institute, whose functions help drive the plot in two of Gersen's adventures.
So in truth, reading The Demon Princes as a revenge tale is like concentrating on where you're going and completely ignoring the scenery along the way. Vance is one of the great descriptive writers in SF, and to concentrate on the plot while ignoring all the cool people and societies he describes is like driving through the Black Hills with your eyes fixed firmly on the Interstate. You could do that, but why bother?