I saw some videos on YouTube in which mother of Vali did request to Hanuman that if in any case there is any situation of war between you and Vali then promise me Hanuman, you won't kill Vali. Hanuman made that promise to her.
One day when battle became necessary between Hanuman and Vali, then Lord Hanuman kept his promise and didn't kill Vali. He was able to kill Vali but didn't do so because of his promise.
Here is the Video link of this story. I am posting link of one episode but you can also watch other episode of the story also to know in detail. You will find link of next episode in top right corner of that page.