‘And working together with him we entreat also that you do not receive the grace of God in vain.'
So as those who are His ambassadors, as those who are ‘workers-together' with Him (compare 1 Corinthians 3:9), we are therefore to plead with men that they do not receive the grace of God in vain. Here it is especially Paul speaking to the Corinthians, and even more especially to those who were opposing him. It is directly they who are in mind. God's unmerited favour has reached out to them through the Spirit, and through His ambassadors, and he is concerned lest it be ineffective. Their very presence among God's people ensures the continuing activity of God's grace towards them. But let them make sure that they have responded and been open to the gracious working of God, or if not let them now respond to His call, otherwise it will be in vain. Lest they be found to be the seed that sprang up, but then withered and died (Mark 4:16-17), or the withered branches that had appeared to have been a part of the vine, but because they had no life had to be cast forth and burned (John 15:1-6), or the man who had built his house on sand so that it collapsed (Matthew 7:26-27).
‘In vain.' Having achieved nothing, being empty, useless.
Others see it as a plea that they ensure that they do not remain stagnant in their Christian lives, that they put into practise the words of 2 Corinthians 5:15-17, so that they have that which is good to present at the judgment seat of Christ (2 Corinthians 5:10). But the next verse might rather be seen as supporting the first. He wants to urge the certainty of their response to the day of salvation.