Is performing the night vigil (qiyam al-layl) then returning to sleep before Fajr valid, or is it better to wake up an hour or more before Fajr for the night vigil?

What the sister asking the question does, performing Qiyam al-Layl (night prayer) after the 'Isha prayer, is indeed Qiyam al-Layl. It is considered among the righteous deeds for which reward is hoped on the Day of Judgment. The Prophet, peace and blessings be upon him, clarified that the best Qiyam is the Qiyam of Dawud (David), peace be upon him, who used to sleep half the night, pray for a third of it, and then sleep for a sixth of it. Qiyam al-Layl is performed two rak'ahs at a time, and if a Muslim fears the dawn, he should pray one rak'ah of Witr to make odd what he has already prayed. Qiyam al-Layl can be performed at the beginning, middle, or end of the night, but performing it in the last third of the night is better and more likely to bring reward, as Allah Almighty descends to the lowest heaven during this time and says: "Is there any supplicant that I may answer him? Is there any petitioner that I may grant him? Is there any seeker of forgiveness that I may forgive him? Is there any penitent that I may accept his repentance?" The most beloved deed to Allah is the one that is most consistent. And however much the sister strives to make her prayer in the last third of the night, this is good, and the reward for her deed will not be lost.