The reason for getting different qualities could be that you don't change the default quality.
In case of the "constant rate factor" and "constant quality" the lower the value, the better the quality. 0 should even mean lossless video.
Yes, I'm aware of the quality issue, that's a simple point, but the problem is completely different. The issue is that no matter what settings I use when rendering with the Voukoder (including h264 8-bit), the social networks I mentioned (Telegram and Instagram) don't recognize the video file. However, if I use Vegas's standard renderer, the video file is recognized and uploads just fine.