It would be "you're vapid", as in "you are vapid".

You got the vapid part right, now let's work on contractions and their related context.

Please, stop this now.

It wasn't the word 'your' that was wrong, as he used 'vapid' as an adjective to 'rebuttal'.

'vapid' might have been wrong, unless it's a contraction of, or synonym for, 'stupidly and annoyingly objectonable'.

Kudos, however, for agreeing that your rebuttal was vapid (as distinct from 'valid', which it wasn't).