I realize this thread is very old but I just wanted to weigh in on this since I came across this thread while researching the issue. When my Virtual Boy arrived back from being solder fixed I noticed that it had a blurry horizontal line in my left display, much like how it’s described in this thread.
It definitely wasn’t a dead pixel, because it was blurry unlike the dead pixels I had before the solder fix. I had tried cleaning the plastic lenses from the outside and inside alike, and then sprayed compressed air onto the mirrors and the areas around them. That didn’t fix the issue, however, I later tried unscrewing the LED PCBs and spraying those with compressed air as well and that ended up fixing it!
So, in my instance, it was likely a tiny speck of dust that had gotten stuck in there while reassembling. If you notice any blurred lines then I advice taking off the bottom of the Virtual Boy and just spraying some compressed air on the mirrors and such. You might have to pray the actual PCBs like I did, but you only need to remove the two screws holding those in place to be able to lift them up and spray them. Fairly simple fix overall. Hope this information is useful to those having similar issues.
