I ordered 4 folding cup holders from Camping World (www.campingworld.com) for about $5 each. They look a lot like the cup holders I've seen in non-Westfalia Vanagons. They attach to a vertical surface. When folded, they're about 3 inches square by 3/4 inch deep. To use them, the front folds down (forming a little "floor"), and a pair of adjustable arm folds up (to hold the cup). They'll hold about any size cup.
Using double-stick tape, I attached one on each side of the automatic transmission shifter housing up front, one to the right side wall just rearward of the sliding door, and one on the left sidewall just over the rear table but below the window. They have worked out well.
The backs of the cup holders really don't have a good flat spot for the double stick tape, so I used multiple layers. I colored the edges black with a permenant marker so they wouldn't show up as much. If I had it to do over again, I'd skip the tape that's included with the cup holders, and use the extra-heavy-duty 3M double stick tape that says it's for "permenant" mounting. The cup holder on the right wall fell off once, and using the extra-heavy-duty tape seems to have fixed it. The ones on the shifter housing look a bit precarious, but they've held up fine - possibly because they're attached to nice hard plastic rather than the softer wall material. The one on the left wall is unlikely to fall, because it actually sits on the table when open.
I've seen very similar but somewhat less expensive cup holders at K-mart, but the backs of those are even less flat and might be even harder to attach double stick tape to. Of course, probably the smart thing is to just screw or bolt the things into the walls, but I just didn't feel like making holes in my walls.
I've seen Vanagons with a pair of these cup holders attached the left side of the "box" that the passenger seat sits on. They block the aisle a bit when open, but if you don't mind that then that seems like a good location, too. You could also stick them to the front doors, but I couldn't find a good spot - the cup holders always got in the way of the window crank, or the arm rest, or something.