A simple thumbs up, means 'I agree ' so in the context of a discussion forum requires little additional input.
Laughing at someone via emoji without providing further context is, both ill mannered but also ambiguous, because its not clear what it is the person is dissagreing with, and therefore has no place on a discussion board, IMO.
Nothing to do with vanity.
Edit. Correction, vanity does play a part, most people don't particularly like being ridiculed, but then you clearly know that already, but do it anyway. Says a lot.