Is failing to fulfill a vow, despite being able to do so, considered major hypocrisy that expels one from the fold of Islam, or only one of the major sins? And what is meant by His — the Exalted — saying: "So He caused hypocrisy to be in their hearts"?

A vow must be fulfilled when its condition is met. Failure to fulfill it, despite being able to do so, is considered a sin and a major transgression. However, it is not an act of disbelief that expels one from the fold of Islam. Hypocrisy in deeds is a sin, whereas hypocrisy in belief is disbelief. The noble verse: "So He caused hypocrisy to be in their hearts until the Day they meet Him, because they broke what they promised Allah and because they used to lie" means that their action caused hypocrisy to remain in their hearts, or that hypocrisy was used to describe committing sins within Islam.