Is it permissible to continue performing two rak'ahs before sleep daily to fulfill a vow, and is it permissible to perform two rak'ahs before the Fajr prayer daily with the intention of fulfilling a vow, and is it possible to combine the intention of performing the Sunnah of Fajr with the intention of fulfilling a vow in a single prayer?

The vow must be fulfilled, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." Therefore, pray two rak'ahs every day before sleeping. As for vowing two rak'ahs before Fajr prayer, it is not valid according to the majority of scholars, due to the prohibition of performing voluntary prayers after the emergence of dawn, except for the sunnah (definition) of Fajr, because a vow is only binding for that which is recommended. The Fiqh Encyclopedia mentioned the disapproval of performing voluntary prayers after the emergence of dawn and before the Fajr prayer according to the Hanafis, its prohibition according to the Hanbalis except for the two rak'ahs of Fajr, and its disapproval according to the Malikis except for the desirable Fajr prayer and the established daily recitation (wird). Therefore, this vow is not valid and cannot be combined with the sunnah of Fajr.