# 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.