How To Force Vertical Full Screen on YouTube Android

Just install this app called Rotation Lock Adaptive (Free) and make sure it's configured to autostart.

AND BOOM! Now when you open YouTube and make it full screen, it will not go in landscape mode, but will stay in portrait mode. How awesome is that? 

That's was so easy! But who would've thought that such a pesky problem would be fixed simply by installing this lightweight app. 

I randomly came to this discovery while seeking to have manual rotation of my phone. Enjoy and share if you found it useful!