You've got the reflected DIC right -- a single prism is used to shear both the epi illumination and then combine it back headed up into the eyepieces. Some Olympus generations used a single sliding prism to cover all magnifications. Other generations used adjustable prisms located on an interchangeable nosepiece. Typically, 10x, 20x, 40x, and 100x magnifications were supported with the individual prisms.
Basic setup is to get the polarizers crossed for extinction and then introduce the DIC prism into the path. If you have all-original components it should be pretty straigtforward. Where it gets complicated is if different generations have been mixed -- the proper orientations of the polarizer, analyzer, and DIC prism can then take a while to sort out.
You need to polarize the light headed down on to the specimen and then polarize (or "analyze") it again above the DIC prism. If you're missing a polarizer or analyzer, you can probably get a high quality linear polarizer and make a blackened aluminum frame for it. Even cheap circular camera polarizers can be used, but they may need to be flipped to become "linear" and they typically don't give as full extinction and as good DIC.
The Vanox is a massive scope, beautifully made, but with all sorts of camera controls that are now obsolete. Chances are you could move the optics to something like an Olympus BHM (stripped of all the now-obsolete controls) if you wanted. If yours came with a set of newer Neo Splans (chrome barrels, not black enamel with color bands) that's good news. If you have the single sliding prism setup, that's also very versatile.