Answered by Shaykh Amjad Rasheed
Does one attain unto the reward of night vigil worship (tahajjud) if one sleeps before Isha time and then gets up in the middle of the night, and prays Isha and a few rakats?
In the Name of Allah, Most Merciful and Compassionate
May His blessings and peace be on His Beloved Prophet, the best of creation, and his family, companions, and followers
Yes, the reward of night vigil worship (tahajjud) is attained if one sleeps before Isha prayer, or even before Isha time (such as sleeping after praying Maghrib) and then gets up at night to worship. This was stated explicitly by Allama Bajuri, and entailed by the words of Shaykh Ibn Hajar in his Sharh al-Irshad, as mentioned by Shabramullasi in Hashiyat al-Nihaya.
[However, performing Isha in congregation is greater in reward, and should not be left for this.]
And Allah alone gives success.
– Amjad Rasheed.
(Translated by Faraz Rabbani)