Does forgetting Surat Al-Baqarah after memorizing it constitute fulfilling the vow, and must it be re-memorized despite a weak memory?

A promise does not necessitate fulfillment, but it is commendable to fulfill it, especially if it is between a servant and their Lord. You have done well by fulfilling your promise to memorize Surat Al-Baqarah, and there is no blame upon you for forgetting it due to a weak memory. You are not required to fulfill the promise again. Memorizing Surat Al-Baqarah is among the best deeds due to what was narrated concerning it from the Prophet, peace and blessings be upon him: "Recite Surat Al-Baqarah, for taking it is a blessing and leaving it is a cause of regret, and the sorcerers cannot confront it."