What is incumbent upon a person who vows to fast a certain number of days if Allah protects him from harm? Does the ruling differ if the harm befalls him compared to if it does not?

If a vow is made conditional on something, it becomes obligatory to fulfill it when the condition is met. If the condition is not met, then nothing is obligatory, for Allah Almighty says: "They fulfill [their] vows and fear a Day whose evil will be widespread."