VLC as evidenced by plethora of complaints on multiple websites, has a very unreliable cast experiences.
VLC by default, automatically searches for cast devices and if it finds any, then shows the cast button. It creates a very bad experience for the user as often VLC fails to find the cast devices and thus never shows the cast button in the app. (You can find lots and lots of reports of people complaining about VLC's cast)
On my own personal anecdotes, VLC has never shown the cast button, I've had reinstalled the app, restarted the device and due to my changing custom Roms, I consequently have also reset my device multiple times, even changing Android version, but VLC has never shown me the cast button. I'd like to point out that throughout this time I still have been able to cast via youtube, mirror my device screen, cast via a third party app ( local cast ). Even during the screen mirror casting when I opened VLC, it still didn't show the cast button in the app. Ha! (Yes wireless casting is turned on in the settings, i check every time)
So thus my proposed solution is that there needs to be a button that opens a menu which force-searches for cast devices every time you press it; Instead of relying on some background process which you can't control and can't effectively use.
It will solve lots of problems that people experience.