What is the ruling on not performing Qiyam Al-Layl due to drowsiness after vowing to perform it daily? And what is its last time? And is expiation obligatory for not fulfilling it? And is forgetting the aforementioned vow a hindrance to the obligation of fulfilling it?

The phrase mentioned is a binding oath, not a vow. If you were an adult at the time of swearing the oath, it is recommended to fulfill it. If you intentionally break it, you are required to pay the expiation for breaking an oath. However, if you missed performing Qiyam al-Layl due to sleep, there is no expiation upon you because you did not intentionally break the oath. If your statement was made before puberty, there is nothing upon you, because there is no expiation for vows or oaths made before puberty. The last time for Qiyam al-Layl is the rising of dawn. If you are unsure whether you broke it or not, there is nothing upon you.