Is it prescribed to point with the index finger and recite the opening supplication (Du'a al-Istiftah) and the subsequent ذكر (dhikr) mentioned in voluntary (nafilah) prayers, just as it is prescribed in obligatory (fard) prayers? And is the opening supplication (Du'a al-Istiftah) required in every two rak'ahs of the night vigil (qiyam al-layl) and Duha prayers?

Everything prescribed for the obligatory prayer is also prescribed for the supererogatory (Nafl) prayer. The opening supplication (Istiftah) is not obligatory in either of them, but rather it is recommended. Thus, it is sunnah to recite the Istiftah after every ihram (entering into prayer) whether it is an obligatory or a supererogatory prayer, with the exception of the funeral prayer and the prayer of someone who joins the Imam late (masbuq) if he catches the Imam in a state other than standing (qiyam). Therefore, reciting the Istiftah in the night prayer (Qiyam al-Layl) and the Duha prayer is recommended at the beginning of every two rak'ahs of them.