So as a starting point, you have three (essentially) vassals - the Landmeister of Livland (who is the successor of the master of the Livonian Order, but that office is essentially defunct at this point I believe), the Landmeister of Prussia (who it looks like might technically be one and the same with the Hochmeister, but for balance purposes I'd make it a separate vassal), and the Landmeister of "The Holy Roman Empire". That's a goofy phrase, but I assume it refers to the lands held in Germany proper, such as those in Pomerania.
Then, you would probably have this as a vassal: http://en.wikipedia.org/wiki/Duchy_of_Samogitia. According to that page, in our time-frame, its a guy named Rumbaudas Valimantaitis. I would figure there'd be one or two Prussian lords under your vassalage too, but I don't know where to find those. I'll keep poking around.