Not bad, but the center spot should be larger. The vignette should start just about where the middle of the cloud is. There is an "Offset" option in GIMP, which makes it skip some distance and then start vignetting.
BTW, I see you oversaturated your image. If you want to make it look like a cross process, you should colour in the vignetted area differently, as usual cross process makes colour shifts different, depending on exposure in different areas.

I think I have it this time.. I had to open up the center more and have less vignette, and I added an additional "radial blur" on the vignette layer to smooth out the featuring.

If you click on the image below, and use the back arrow, you can the change I made from my previous posted image.

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Thanks, You are right, not all subject matter works for the LOMO effect...More open framing and paying attention to what is in the center helps.

I am still trying to figure out how to have less vignette intruding... but, I will. I think I have an idea, I noticed that too...

We are both making a mistake applying linear gradient, while it should actually be an exponential one.

What is that?

I have a preset vignette that allows some control on the amount of vignette and what kind of blend to use, though limited compared to what is available on a layer.

Not to be overly negative, but its a bit hard to fake some of these effects in my opinion.  Blurriness around the edges should still be affected by the distance that different objects are at and is (almost) never a gaussian blur, and is heavily affected by lighting that is outside the range of the image (a bright point light will make a circle/other shape on an out of focus image, a faked blur will average it out because the brightness is clipped).

And in the case of this image, the cloud is clipped to full, so the vignetting that's getting applied looks fake because its getting turned to flat grey instead of regaining detail...


Oh, and also should probably add some soft focus to these images too.  That 'dreamy' plastic lens on Holgas is a major part of the look!  Again, its something that is affected by brightness level outside of the range of the image, but not as critical for most images... I think

Obviously there's no right or wrong in any kind of art form. I'm just saying since you're going for the Holga look, there are some things to take into account that can make it very difficult to achieve.  I think it's easy to get an image that somehow looks 'wrong' when editing something because our brains are wired to expect things in ways we understand them to be.  So, even small discrepancies pop out and, unless it's the intent of the artwork, it can be distracting.

That said, its definitely a good exercise in image editing, and I kind of want to take a crack at it now too... if I weren't at work. Hehe.   

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  Blurriness around the edges should still be affected by the distance that different objects are at

Indeed, curvature of field would do that too.

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never a gaussian blur

That's why I'm applying "zoom" blur, in order to mimic the coma/astigmatism they produce around the corners.

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and is heavily affected by lighting that is outside the range of the image (a bright point light will make a circle/other shape on an out of focus image, a faked blur will average it out because the brightness is clipped).

I haven't god a recipe for that yet. The shape of the bright circle will also be affected by coma/astigmatism as stated above, so it will be very difficult to mimic in PP.

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Obviously there's no right or wrong in any kind of art form.

Sure, we are having fun and trying to create something we like.

RE RED ^^^

I have also found that a "zoom" blur works better for the corner distortion.. Also a slight "Twist" blur is interesting..

Twist Blur With the .45x Wide Converter on the O/14-42ED at 17/18mm (100* FOV) to have slight vignette at the time of capture.

Wheeeee, my attempt. 30 minutes on Photoshop this morning before work.

Original:

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Holgaized:

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Things done:
  * Chromatic aberration: Split RGB into different layers, distorted blue and red with barrel/pincushion) with a mask so it doesn't affect the center as much. Thinking back, I think it would have made more sense to adjust the size of the red and blue instead, as these distortions don't effect the corners hardly at all, and those areas should be the most affected.
  * Soft focus: Copied image, heavy gaussian blur, and 'screen' on top of the original, adjusted opacity until I liked it
  * Blurred outside: 'Zoom' blur
  * Vignette: Radial gradient on top layer with 'multiply' effect. Played with brightness and contrast until I liked it, which changes size and 'quickness' of the falloff to black.  Ended up removing the blackness out of the upper right corner because I didn't like it... maybe should have removed from upper left too.  From looking at the pictures from my Holga 135BC ('black corners' ie specifically made to vignette the pictures) depending on the situation it doesn't always vignette, and not always all the corners.  I still don't really like it though... probably would be best to do an exponential curve like Adash said... maybe would make sense to play with the curves in Photoshop instead of just brightness/contrast.
  * Brightness/Contrast: Layer on top, made darker since the soft focus screen makes it lighter, and added contrast to make it more punchy.