Load mp4 files in Sony Vegas that cannot be opened

This is how you open an mp4 file, perhaps from YouTube, in Sony Vegas when Vegas tells you “none of the files dropped on Vegas Pro could be opened.”

Using VLC, open mutliple files using Control-Shift-O.

Drop the offending video on the File selection box.
 

Click the arrow next to the Play button and choose Convert.

On the Convert dialog, set the Profile to H.264 + MP3 (MP4).

Click the small wrench button next to the profile selection to edit the profile.

On the Encapsulation tab, make sure the format is MP4/MOV.

On the Video codec tab, check Keep original video track.

On the Audio codec tab, check Keep original audio track.

Click Save and choose a Destination file.

The save will occur very quickly because only the wrapper is changed.