How to limit audio volume on iPhone for all apps?
iOS handles volume differently depending on the task at hand. For example, the volume for playing games is set differently to the volume for playing videos.

In your example, YouTube and VLC are both for playing videos. If you need the audio level for these types of apps to be lower (or off) you need to play a video and then lower the volume. Once you've done this, iOS remembers this for all video playing apps (i.e. turning it down on VLC will mean that it's also down for other video playing apps).

You just need to remember then that even if you turn your volume up while using the phone, that when you next play a video it will still be at the level it was at when you last played a video.

Hope this helps.