Albert Barnes Commentary 2 Kings 17:15

"And they rejected his statutes, and his covenant that he made with their fathers, and his testimonies which he testified unto them; and they followed vanity, and became vain, and [went] after the nations that were round about them, concerning whom Jehovah had charged them that they should not do like them." — 2 Kings 17:15 (ASV)

As idols are “vanity” and “nothingness”—mere weakness and impotence—so idolaters are “vain” and impotent. Their energies have been wasted, their time misspent; they have missed the real object of their existence; their whole life has been a mistake; and the result is utter powerlessness. Literally, the word rendered “vanity” seems to mean “breath” or “vapor,” a familiar image for nonentity. It occurs frequently in the prophets, and especially in Jeremiah (for example, Jeremiah 2:5; Jeremiah 8:19; Jeremiah 14:22; and elsewhere).