Is it permissible to perform the night vigil (Qiyam al-Layl) prayer with Witr after the first call to Fajr prayer (adhan), given that there is sufficient time to perform the Sunnah of Fajr?

It is permissible to perform Qiyam al-Layl (night prayer) and Witr (odd-numbered prayer) in the latter part of the night. It is preferable to perform them in the last third of the night, based on the hadith of the Divine Descent. If the questioner's intent with "the first call to Fajr prayer" is the adhan of the sixth part of the night, then this is one of the best times for worship, and it is permissible to pray during it until the adhan of the true Fajr. After that, no supererogatory prayers are performed except the two rak'ahs of Fajr. And whoever sleeps through their nightly wird (recitation) or Witr, they may pray them after sunrise and before the Fajr prayer, as long as they do not fear the time expiring or missing the congregational prayer.