Should I pray the Night Vigil (Qiyam al-Layl) and partake of the pre-dawn meal (suhoor) for voluntary fasting according to the শরিয়া-prescribed time, or should I wait for the adhan from mosques that delay the prayer by a full hour?

The time for performing Qiyam (definition) al-Layl (night prayer) begins after the 'Isha prayer and ends with the emergence of the true dawn (Fajr Sadiq). The most preferable time for it is the last third of the night. Prayer and suhoor (definition) (pre-dawn meal) are permissible until the time of the true dawn, as Allah Almighty says: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night] from the Fajr." The decisive factor is the emergence of dawn, not the adhan (definition) if it is delayed. One can use reliable calendars for assistance.