What is the ruling on one who vowed to maintain prayer, then abandoned it for a period exceeding nine months? Does this necessitate an expiation from him?

You must offer sincere repentance for abandoning prayer, as it is a grave sin. You are also obliged to make up the missed prayers because they are a debt upon you. As for vowing to abandon prayer, the majority of scholars hold that such a vow is not binding and does not require expiation. However, you may act cautiously and offer expiation for an oath.