I had an issue with Voukoder ever since Vegas 20 build 403, on my ancient Xeon X5670/Radeon HD6870 setup, where Vegas would crash as soon as I attempted a render with the Voukoder plugin.
I'm pleased to say that Voukoder Pro doesn't have this issue
Configuring a scene with LIBOPENH264 video and AAC audio muxing into mp4 seems to yield a video file which shows the same codec info in VLC as my old videos from back when Vegas hadn't broken Voukoder for me. The only setting I had to tweak was the AAC bitrate, taking it up to 128.