Israel exchanged the reality of God for the falsehood, the vanity, of idols (Jer. 2:5). They "exchanged the truth of God for the lie" (Rom. 1:25a). All idols are vanity; thus, to pursue an idol is to pursue vanity.
Israel worshipped and served the creation rather than the Creator (Rom. 1:25b). They worshipped and served not only the things of God's creation but also the things of their own creation—idols of wood, stone, and metal.
Israel sought the Egyptians' and Assyrians' help (man's help) instead of seeking God's blessing (Jer. 2:17-18, 36; Isa. 31:3). They did not trust in God's blessing but in man's help.
As the wife of Jehovah, Israel forsook her chastity to commit fornication, having a harlot's forehead and refusing to be ashamed (Jer. 2:20, 23-25; 3:1-3).
Israel practiced apostasies rather than keeping her faithfulness (2:19). Apostasy is a matter of leaving the way of God and taking another way to follow things other than God. This is what Israel did.
Israel increased the number of their gods (idols) according to the number of their cities (vv. 27-28). Wherever there was a city, there was an idol.