The doc for the Convert/Stream... feature says:
In VLC, go to "Media >> Convert / Save..." (Ctrl+R).
Add the input file(s) or stream(s) you want to transcode. Press "Convert / Save".
In the "Convert" windows specify the destination file name (including file extension - e.g. on Windows "C:\out.mp3").
Select the desired codec from the profile list. To check/edit the settings of a given profile press the "Edit selected profile" button. Profiles can also be added/deleted.
Press "Start".
In macOS, "Convert/Stream..." is under the File menu, but OK. I choose that, pick the input media, pick the Profile, choose the destination... and there's no Start. If I choose Save (the only option), the source gets added to the current playlist, and a placeholder file is written to the destination. Clicking Play for the playlist results in a message that the output file exists, and no conversion occurs.
So what's the correct process for converting (e.g.) an AVI into an M4V?
EDIT: Sorry, forgot to include context. VLC 3.0.0 on macOS 10.12.6 on a MacBook Pro (MacBookPro13,2) with 16GB.