Is it obligatory to re-memorize Surat "Maryam" after forgetting it, when its memorization had been vowed and completed?

If you have memorized the surah you vowed to memorize, you have fulfilled your vow, and your obligation is discharged. You are not required to make another vow if you forget it, unless your vow explicitly included not forgetting it. Nevertheless, you must review what you have memorized from the Qur’an because forgetting it is a sin.