Is this the Voigtlander Vito C seen in a photo in the new toy thread? If so, that looks too small to have shutter curtains, which technically means a focal plane shutter with either cloth or metal blinds. If it has a leaf shutter (which would be shutter blades rather than curtains), then it's pretty unlikely to have a leak there. Cameras that have cloth curtains in a focal plane shutter can end up with damage in the form of tiny pinholes, but it only tends to affect non-SLRs (usually rangefinders with interchangeable lenses). With no SLR mirror to act as a barrier, the lens is always projecting light onto the shutter. If the lens is at a particular focus distance, an object at infinity can be rendered as a sharp image on the curtain rather than on the film plane. If that object happens to be the sun, then there's a risk that the curtain can get burnt, resulting in a little pinhole (or pinholes if it happens several times with the sun in various positions with respect to the frame). If your camera doesn't have a cloth blind focal plane shutter, then this pinhole issue shouldn't be a factor. Having looked at some photos of them (but none showing the inside), it looks too small to have such a shutter.
It's possible that the blades in a leaf shutter can be a bit iffy and not always close correctly, but I don't think that would result in such a well defined pattern. It would, in effect, be more like having the lens set to a small aperture, the opening of which happens to be off centre. I believe that would result in a dull projected image, probably with vignetting towards the centre of the frame. In other words, it would be bigger and have less defined edges.
I think the shutter can be eliminated as a possible cause. That leaves some sort of opening in the body. Possibilities that come to mind are: the front of the viewfinder, the back of the viewfinder, the rewind crank, maybe something around the side of the lens barrel, and possibly the back cover depending on what the pressure plate looks like. It should be possible to narrow it down by a process of elimination, and you can use the film as a detector (which is handy).
Does the light leak extend outside the frame when you look at the negatives? By that, I mean well into either the sprocket hole area or the vertical space between frames. If it does, then the leak is more likely to be coming in from behind. If it doesn't, it's coming in from the lens side of the frame.
You say it's intermittent. Possible reasons for that would include: the light path inside the camera changes because something inside is moving around (so sometimes blocked, sometimes open), how you hold the camera could sometimes block it from the outside or reduce the intensity of the light coming in, and the ambient light itself could sometimes be too dull for the leak to be noticeable.
Give the camera a shake in various orientations next to your ear - any little rattles? If any external bits make noises, hold it in such a way as to keep them still.
Does the intensity of the leak vary? The two examples here look pretty similar - are there duller versions?
Can you correlate instances of the leak with usage patterns? For example, if you've had the camera closed for a while, does the first shot after that tend to have the leak? Or does it happen when the camera has been open for a while but a shot hasn't been taken? If you take a few shots in quick succession, do they have the leak? Does it appear in shots that are framed vertically? How do you hold the camera that's different from horizontally oriented shots - ie, which parts of the camera body are covered in one orientation and not the other, and which parts are covered in both orientations? When vertical, is it held right hand up or right hand down?
You can do a series of tests in a dark room to try to force the leak to appear (this is where your film is used as a detector). Find something light tight that can be used to cover the various bits of the camera that are potential paths for light to get in. Tape can be used for some bits. For others, like the viewfinder, maybe tape with little bits of card to stop the adhesive getting onto the glass. In each case, try to cover everything that might be a possible light path. Start with the front door closed. Wind on to a new frame and bathe the camera is strong light. If using a troch, move it around to direct light at the camera from all angles. A more diffuse source like a desk lamp is better than a torch - using it fairly close to the camera will improve the chances of getting light in at the right angle (daylight is diffuse).
After each bathing of the camera in light, switch the strong light off, fire the shutter, advance the film, and remove one bit of the covering (if you've taped up the rewind crank, remove the tape from that first so that it can rotate). When it comes to opening the front for bathing in bright light, use tape/card to block off the front of the viewfinder while leaving the lens barrel clear., then do another step with the viewfinder clear. Repeat the process for each possible path, firing the shutter and advancing the film each time. (set the film speed as high as it will go to ensure minimal exposure of the image to allow any leaks to be more obvious). If you do this in a dark room, you should be able to open the front to expose the frame without introducing the leak to a great extent.
Take notes at each step so that you have the sequence of which part of the masking was removed. If you make a mistake, like opening the front to expose a frame while there is a bright light present, fix the error (kill the bright light or whatever) expose the frame, advance the film and repeat the step. Note down the occurrence of the mistake.
If the leak is being caused by an opening in the body, it shouldn't be too hard to track it down - all you have to do is point the light at the camera for a reasonable length of time at each step.
It's on quite a lot of frames bottom right hand corner as you look at the image. Do these things have light seals at all? There is slack in the way the back shuts so you can move it very slightly side to side and the plastic squeaks a little. It was ok before. It's just started doing it in the last roll. It was ok earlier in the year but on very sunny days it seems it does let more light in that it should.
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