Charles Ellicott 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)

And they followed vanity, and became Vain. —The same expression occurs in Jeremiah 2:5. The word “vanity” (hèbel) has the article. It denotes strictly breath; and then that which is as transient as a breath. (Compare to Job 7:16.) Here the idols and their worship are intended. The cognate verb, “became vain,” means “dealt (or, ‘talked; ’Job 27:12) foolishly.” The Septuagint has ἐματαιώθησαν. (Compare to Romans 1:21.)