Actually many people encountered the same thing with official build of avs4x264 in MeGUI. I'm not a developer of MeGUI. I use avs4x264 as a cli tool only because it is convenient and does not require many tweaks in command line like other pipe tools, and I have never used it with MeGUI ( though I decided not to change the behavior of avs4x264mod so that it can safely replace avs4x264 in MeGUI ). So I have no idea why in some cases MeGUI uses vfw4x264 while in other cases it uses avs4x264. Maybe asking a developer of MeGUI would be helpful