"'Those who cling to worthless idols forfeit the grace that could be theirs. But I, with a song of thanksgiving, will sacrifice to you. What I have vowed I will make good. Salvation comes from the Lord.'" NIV translation
Grace (God giving us what we do not deserve) is evident throughout Jonah chapter two. God gave Jonah a second chance and, as we see in this passage, Jonah had settled in his heart that he would listen to God. He must have made a vow as he was dying something like: If you let me live, I will go to Nineveh as you have said. Jonah did not deserve any second chance but God granted it to him and he promised to listen and do what God had told him. When we come to Christ for salvation, we also vow to turn control of our lives over to Him (make Him Lord as well). Like Jonah, when we make a vow to God we should not do so lightly but we should count the cost and be prepared to keep the promise.
"And the Lord commanded the fish, and it vomited Jonah onto dry land." NIV translation
What a picture we have here of the resurrected Christ. Jonah, who was dead, is brought back to physically walk the earth. Many years later, Jesus would be resurrected and physically walk the earth for a time teaching his disciples. It is also comforting to know that God can use anything even a big fish to accomplish His purposes. If he can use a big fish, he can certainly use you and I.