Question: how to convert rmvb to mp4 using vlc?
Answer: Open VLC > Media menu > Convert/Save.
Add a RMVB file and click Convert/Save.
Select the default MP4 profile “Video – H. ... 
Specify the output file in “destination file” and click Start button. ... 
Wait for the playback time slider to move to its end – that moving slider indicates conversion in progress.