That I’m pretty sure is a colourspace/gamma correctness problem, probably in the underlying libraries we use, but it is annoying and does need investigating and correcting. It’s apparent in the vignetting and other alpha feathering operations too, and not at all specific to vector graphics (it’s a raster compositing problem).
See figure 14 here: What every coder should know about gamma | John Novak
[It’s a Well Known Problem, from the same class of “Well Known” I’ve been cringing from here 
An easy mistake that lots of people innocently make until they have enough of the right bruises to know better.]
So we might not be able to fix it in kdenlive, but this is as good a place as any for someone to start investigating it from - so if there isn’t one already, please do open a report in the bug tracker for this one too. Feel free to quote the above, including that reference and a screenshot from a suitable frame of your own video.
We don’t use this forum for tracking bugs, but it is the right place to triage “I’m not sure if this is a bug” and “I wonder if this idea I have would be feasible” questions.