I need to disable VFR for H264 encoder (Intel Media SDK, Windows). I have no ideas how to do that, don't see simple option to make constant frame rate like in other encoders. I'm using DirectShow filter sample, but I think this question is about encoder core, not DirectShow.
The directshow samples are deprecated. As samples, even before they were deprecated they were released as starting points and not intended to be complete. You are encouraged to change the h264 filter to meet your needs. However, since the sample is deprecated our ability to help with it will be limited.
The best place to start to learn how the Media SDK encoder works is with the tutorials: https://software.intel.com/en-us/intel-media-server-studio-support/code-samples
For encode, you can start with simple_3_encode.
The default for Media SDK is constant frame rate. It's more complicated to do VFR, since this involves timestamps and (possibly) bitrate control, GOP structure, etc.