I usually read Cwmru Fyw on my tablet. I have never seen the VOCAB button before but I can use it on my laptop and it’s amazing. No more switching back and for to Google translate. This should improve my vocab no end. Thanks for pointing it out.
As their (BBC Cymru Fyw’s) answer says, you can use it on your tablet if yu read through web browser rather then their app. In browser the button VOCAB appears and works the same as on PC.
However, to be honest, I didn’t try and research this yet but I might one of these days.