Prayer after 'Isha' is from the Night Prayer
Question :
Is the prayer of one who offers eleven Rak'ahs immediately after 'Isha' considered to be Qiyamul- Lail ?
Answer:
Qiyamul-Layl takes place when one prays a lot, such as for two or three hours, whether the number of Rak'ahs be great or little, and whether it be performed in the early part of the night after 'Isha', or the latter part of the night before Fajr. But the best time is in the third part of the night, after waking from sleep; and this is achieved by sleeping in the early part of the night.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 320-321