How is the ‘Isha prayer performed, and is its specific Witr prayer performed before or after the Night Vigil (Qiyam al-Layl), and is it permissible to delay the ‘Isha prayer until after midnight and before Fajr, and can it be delayed to be prayed with the Night Vigil?

The 'Isha prayer is four rak'ahs when resident and two rak'ahs when traveling. Its validity does not depend on praying the Sunnah after it. The emphasized Sunnah for 'Isha consists of two rak'ahs after the obligatory prayer, and anything beyond that is part of the recommended night vigil (Qiyam al-Layl). The Qiyam al-Layl prayer is performed two rak'ahs at a time, concluding with one rak'ah of Witr.

Congregational 'Isha prayer is obligatory for men. The time for 'Isha prayer begins with the disappearance of the red twilight and extends until midnight (time of choice). The period from midnight until Fajr is a time of necessity. Therefore, congregational 'Isha prayer is obligatory. If you miss the congregation due to an excuse, pray it within its optional time. If you miss the optional time due to an excuse, pray it during the time of necessity after midnight. After 'Isha, pray its regular Sunnah (two rak'ahs), then pray as much of the night vigil as you wish, and conclude the night prayer with Witr.