Ran into this problem with h.264 MP4 files recorded and remuxed in OBS (using current version 30.2.3). Vegas 20 will crash when trying to render the project through Voukoder or VoukoderPro. Even opening a single OBS-generated h.264 MP4 file and trying to render it causes the Vegas 20 crash. No helpful log files from Voukoder or VoukoderPro, seems like it is Vegas itself that is crashing. Crash does not occur when using non-Voukoder encoders in Vegas (e.g., Sony AVC/MVC). Crash also does not occur with h.265 MP4 files from OBS.
Tried turning on/off the Legacy AVC/HEVC decoding and MKV Reader in Vegas 20 (Options -> Preferences -> File I/O). None of these settings made a difference.
Searched the forums, and found this thread.
The solution, as noted by Joelson_Forte above, was to reassert the framerate in Vegas Project Properties (File -> Properties), even though it appeared to be set correctly to 23.976. I pulled down the Frame Rate menu, and selected the same 23.976 option that was already selected. Hit OK, and now the project will render.
Voukoder v13.4.1, connector v1.7.1, VoukoderPro v1.3.0