1. Can namaz-e-tahajud be prayed after vitar?? 2. Is Isha namaz complete/accepted without vitar??

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

According to the view of most scholars the Witr prayer is a confirmed Sunnah. However, some scholars are of the opinion that it is an obligation to perform the Witr prayer, but the correct opinion is that it is a sunnah whose reward is great as the Prophet used to perform it.

For more benefit in this regard, please refer to Fataawa 83303 and 86938.

Moreover, the Witr prayer is not a condition for the validity of the ‘Isha prayer. However, it is desirable that the night prayer be ended with Witr as ‘Abdullaah Ibn ‘Umar narrated that the Prophet said: "The prayer of the night is in twos and twos, and if one fears that Fajr (dawn) would break (before having performed his Witr prayer), then he should finish his night prayers by performing one Witr Rak'ah." [Al-Bukhari and Muslim]

Nonetheless, if a person prays after having performed the Witr prayer, then this is permissible because it is confirmed that the Prophet prayed two Rak’ahs in a sitting position after having performed the Witr prayer, and the Prophet did so to show that this is permissible, as mentioned by the scholars . Nevertheless, one should not do this on a regular basis.

For more benefit, please refer to Fatwa 83045.

Allaah Knows best.