C. F. Keil and Franz Delitzsch on 2 Kings 17:15

2Ki 17:15 “They followed vanity and became vain:” verbatim as in Jer 2:5.

A description of the worthlessness of their whole life and aim with regard to the most important thing, namely, their relation to God. Whatever man sets before him as the object of his life apart from God is הבל (cf. Deu 32:21) and idolatry, and leads to worthlessness, to spiritual and moral corruption (Rom 1:21). “And (walked) after the nations who surrounded them,” i.e., the heathen living near them. The concluding words of the verse have the ring of Lev 18:3.