I say, sayest thou - In 2 Kings 18:20, this is ‘thou sayest;’ and thus many manuscripts read it here, and Lowth and Noyes have adopted that reading. So the Syriac reads it. But the sense is not affected whichever reading is adopted. It is designed to show to Hezekiah that his reliance, either on his own resources or on Egypt, was vain.
But they are but vain words - Margin, as Hebrew, ‘A word of lips;’ that is, mere words; vain and empty boasting.
On whom dost thou trust, that thou rebellest against me? - Hezekiah had revolted from the Assyrian power, and had refused to pay the tribute which had been imposed on the Jews in the time of Ahaz 2 Kings 18:7.