# the grace of God I am what I am
God's grace or kindness has made Paul as he is now.
# his grace in me was not in vain
"because he was kind to me, I was able to do much good work"
# the grace of God that is with me
Possible meanings are 1) this is literally true, and God actually did the work and kindly used Paul as a tool or 2) Paul is saying that God was kind to let Paul do the work and to make Paul's work have good results.