What does it mean that the creature was subjected to vanity in hope?
The creature was subjected to vanity—the curse—not willingly but by God, who subjected it in hope. That hope is God's promised remedy through the Redeemer, the Son, the seed of the woman, and the coming day when the curse will be lifted.
Les explains that when the curse fell, present-day ecology was set up, so one species feeds on another—leopards stalk deer, birds rob other birds' nests. That is not how God created it; it is the result of the curse. Romans 8 says all creation is aware it was subjected to the curse, but also that the day is coming when the curse will be lifted. Paul writes by inspiration that the creature was made subject to vanity not willingly, but by reason of him who subjected the same in hope. Les says as soon as the curse fell, God came back and said He had a remedy: through the Redeemer, through the Son, and through the seed of the woman.
For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope,