I created the video in VP and saved it. It was automatically transcoded to gif by gfycat.
The video hosted on gfycat and set to loop also shares this problem, only in the gif it's much more pronounced.
This isn't really what needs to be done. The VP output has to be a single loop for imgur/coub/gfycat to use. It seems to me the previous posters wanted the same...
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Actually it seems that I was using an antique version. I've tried to use a new VP version and it doesn't add a black frame. However, I now have no idea how to crop a video without zooming in (I've cut off parts of a 960x540 video and the output result either has black bars, or is too 960x540)
I guess it's a different problem now