I’ve found three approaches from playing with VLC just now. I have tested all three of these and they all work.
Detach video from interface
Preferences -> Interface -> Untick “Integrate video in interface”.
Then, play a video and put it on any output you want.
Clone output
Tools -> Effects and Filters -> Video Effects -> Advanced -> Clone.
Put the clone output on any display you like.
Web interface
Set a password under Preferences -> All -> Interface -> Main interfaces -> Lua -> Lua HTTP.
then
View -> Add interface -> Web
Then, point a browser at http://localhost:8080 and log in with no username and the password you set above and you’ll get a web interface for controls.