I have a couple of these fine cameras, your problem could be a pinhole in the bellows, but I found on both of my Vito's that the catch on the door needs to be checked as sometimes they don't quite close tight, I guess wear over many years, so make sure that the catch is pushed closed, to check for pinholes put a small torch in the open camera in a darkroom, they will soon show up, and can be repaired with liqued electrical tape, or fabric paint
Richard