Two potential issues:
If you cannot launch the Voukoder UI, you're missing the Visual C++ Redistributable dependencies and/or FFMPEG v8.
If you can launch the UI, but it simply doesn't appear in Vegas' Render UI, please check the image for the potential reason.
Vegas' Render UI has an option in the upper right to filter the Render presets, and I believe it filters out Voukoder by default.