I suggest: "Vanity" describes either Adam's sin [ the first recorded sin] or God's reaction to that sin, the curse of Gen 3.
The A.V. always translates "mataiotei" as "vanity". It comes in three places:
1) In Eph 4:17 vanity, the hardness of human hearts not the ground, sounds like a human attitude, sin, rather than God's response to sin.
2) In 2 Peter 2:18 again sounds like sin rather than God's response.
3) In Romans 8:20 it is more ambiguous. Hence the reason for this question.
In 1 Peter 1:18 we are not redeemed from the curse but from the our vain [mataios] conduct which brought the curse.
In Romans 1:24 God did give them over to uncleanness, but, before that happened they were already vain. Rom 1:21. Mataios again seen as a starting point.
God's cursing is not vain. Nothing God does is vain.
The curse did not render Adam and Eve's efforts futile, but difficult.
What is vain/futile is that man should not obey God's spoken commands as in Gen 2:16-17 and other commands e.g. the Ten Commandments.
If I am right that "vanity" is sin and not God's response to sin, does, "for the creature was made subject to vanity" challenge this conclusion?
The Westminster Confession of Faith says that God is not the author of sin thus making something without Him a first cause (see John 1:3). God is holy and uncreated, sin is created. The uncreated does not become created by creating that which is other than Himself, namely, sin. This question is based on New Testament references to one Greek word. It seeks to compare two specific possibilities.