Is a person deprived of success in exams because of breaking a vow to abandon a sin in exchange for success, despite having performed expiation?

The questioner must repent and feel remorse for breaking the covenant, which is considered a sin. Sincere repentance is a cause for divine success (tawfiq). The questioner should focus their fear on not being granted divine success in obedience and on Allah's punishment, and not solely on worldly trials.