I think the issue with Vegas previewing goes beyond Voukoder. Preview since vp18 is for a full-range display, best matching projects set to full-range. Renders are generally video-range. To view video-range renders to match the Vegas preview, they have to be converted back to full-range by the viewer. I do this with MPC-HC by adding the 16-235 to 0-255 preset to its Active pre-size shaders list. YouTube auto-converts uploads to full-range. So YouTube also matches the Vegas preview.
Another possibility is to render full-range. Those will match the Vegas preview without a view transform. But if you upload those renders to YouTube, it will auto-convert them even though they're already full-range. And it won't look good.