Seems as if this has come up in a thread before, because I remember looking into it a bit (no pun intended!)
I think what you're seeing is a reflection off a field lens inside the viewfinder. If you hold the camera a foot or two away from your eye, with the eyepiece facing toward you, and look into it from a 45-degree angle off toward the left (rewind) end of the camera, you'll actually see yourself reflected in a thin strip visible along the right side of the eyepiece.
If it were just light entering through the eyepiece that got reflected, it wouldn't be an issue, because normally your head would block the eyepiece. But by experimenting a bit, you'll see that you can ALSO see reflections that come in through the front viewfinder window at an extreme angle from the left. The reflection seems to be strongest from light sources that come in from almost straight to the side, perpendicular to the way you're pointing the camera. You'll see that if you block the light source with your finger at the side of the viewfinder window, the reflection goes away.
In a perfect world, Voigtlander would have designed in some kind of internal baffle to block off these reflections, but I suspect that with the compact design of the RF/VF module there just wasn't room for one.
So one solution, as you suggest, is to make an external baffle out of tape or something. You wouldn't want to actually cover up any of the viewfinder glass; what you'd need is a little fin that would stick straight out from the front of the camera, to block light coming in from the side. I doubt that it would need to stick out more than about 1/8 inch. A really crafty craftsman might be able to make a 'sunshade' that would run around the top and both sides of the housing that encloses all three front windows -- acting as the viewfinder equivalent of a lens hood. (I'm not sure how you'd attach such a thing, though.)
I'm nowhere near that dedicated, and I seldom find myself in situations where the reflection really bothers me. If it does, I just stick out the tip of one of the fingers that's holding the left end of the camera, to block the light. Or I shift my eye slightly to the right, which causes the edge of the eyepiece to block out the reflecting surface.
