What is the ruling on one who made a covenant with Allah to perform the night vigil (qiyam al-layl) then became unable to fulfill his covenant due to excessive flatulence or insomnia? And does the night vigil remain an obligation upon him, or does it revert to being a supererogatory act?

If the mentioned vow was merely a thought (a feeling within oneself) without being uttered, then nothing is incumbent upon him. However, if it was uttered verbally, then it is considered a vow (nadhr) or an oath (yameen). If illness prevents one from performing Qiyam al-Layl (night prayer), it is recommended to firmly intend to do so, and it is hoped that Allah will record the reward for him, as the Prophet, peace and blessings be upon him, said: "When a servant falls ill or travels, the like of what he used to do when he was healthy and resident is written for him." A Muslim should intend to perform Qiyam al-Layl before going to sleep. Qiyam al-Layl is a confirmed Sunnah (recommended practice) and not obligatory. If one is overcome by drowsiness, the Sunnah is to sleep until the drowsiness subsides, in order to perform the prayer with khushu' (devotional humility).