The Vitr Prayer

its not wajib, because the Prophet (saw) said this....

“Allah has prescribed five prayer in the day and the night.” A questioner said, “Am I obliged to do more?” He said: “No, unless you perform supererogatory prayers.” The man went away saying: “By Allah, I will not increase or decrease beyond these obligatory prayers.” Then, the Prophet (pbuh) said, “He would succeed if he did what he said.” [Muslim]

some people say imam abu hanifah (rahimullah) was of the opinion it was fardh, but the majority of scholars said its mustahab, like highly recommended

and its done after isha, like you said and it can be done till like before fajr time,

check this bro!! ....

http://islamqa.com/en/ref/46544

Witr Prayer, its Time and Ruling

1132. `Ali (bin Abu Talib) (May Allah be pleased with him) reported: The Witr prayer is not obligatory as the prescribed Salat (prayers), but the Messenger of Allah (PBUH) observed it as his regular practice (Sunnah). He (PBUH) said, "Allah is Witr (single, odd) and loves what is Witr. So perform Witr prayer. O followers of Qur'an, observe Witr (prayer).''
[At-Tirmidhi and Abu Dawud].

Commentary: "Witr'' literally means odd number. "Allah is Witr'' means He is One in Attributes and Actions and has no equal. Witr prayer is also called Witr for the reason that it is performed in one, three, five and seven Rak`ah. It is not permissible to perform it in an even number, like two, four, six, eight etc. Thus, we learn from this Hadith that Witr is not Fard and Wajib but Sunnah Mu'akkadah (compulsory). But it would not be correct to show any slackness in performing them to this reason because every Muslim should do his level best to follow the Sunnah of the Prophet (PBUH).
1133. `Aishah (May Allah be pleased with her) reported: The Messenger of Allah (PBUH) observed Witr prayer in every part of night at the beginning, middle and at the last part. He (PBUH), however, would finish his Witr prayer before dawn.
[Al-Bukhari and Muslim].
Commentary: This Hadith tells us the timing of Witr prayer. Its earliest time is soon after `Isha' prayer and its time is just before dawn.
1134. Ibn `Umar (May Allah be pleased with them) reported: The Prophet (PBUH) said, "Make Witr prayer the last of your Salat at night.''
[Al-Bukhari and Muslim].
Commentary: Some `Ulama' are of the opinion that after performing the Witr prayer, it is not permissible to perform any other Nawafil prayers because the Prophet (PBUH) ordered to make it the last Salat. But Imam An-Nawawi and some other scholars have interpreted it as "desirable'' rather than an order, because we do find instances in which the Prophet (PBUH) performed two Rak`ah Nafl prayer in the sitting position after Witr prayer. It is, therefore, better to go by this Hadith. But if someone wants to perform two Rak`ah Nafl prayer after Witr, prayer it is permissible.
1135. Abu Sa`id Al-Khudri (May Allah be pleased with him) reported: The Prophet (PBUH) said, "Perform Witr prayer before dawn.''
[Muslim].
1136.`Aishah (May Allah be pleased with her) reported: The Prophet (PBUH) used to perform his voluntary prayer at night (i.e., Tahajjud prayer) while she was sleeping in front of him; and when the Witr prayer was yet to be observed, he would awaken her to perform her Witr prayer.
[Muslim].
Commentary: This Hadith makes the following three points:
1.It is permissible to pray with someone sleeping in front of us.
2.It is desirable to awaken one's own family members for Nafl prayer.
3.One can perform, Witr prayer before Fajr prayer.
1137. Ibn `Umar (May Allah be pleased with them) reported: The Prophet (PBUH) said, "Hasten to perform the Witr prayer before dawn.''
[Abu Dawud and At-Tirmidhi].
1138. Jabir (May Allah be pleased with him)reported: The Messenger of Allah (PBUH) said, "He who apprehends that he may not get up in the later part of the night, should observe the Witr prayer in the first part of it; and he who is certain to get up in the last part of it, he should observe Witr at the end of the night, because Salat at the end of the night is attended (by the angels), and that is better.''
[Muslim].
Commentary: If a person is confident that he will get up to perform Witr prayer before dawn, it is better for him to perform it in the late hours of the night; otherwise, it will be well to do so after `Isha' prayer.

Source.

Witr prayer, according to hanafi mazhab, is wajib. Which means that if you miss any, you have to do Qaza for it.

It is prayed in three rikaat; sitting between the second and third rikaat, then getting up, reciting fatiha and a short surah, then you say takbeer and raise your hands, then fold them back and recite qunut. After, qunut you say takbeer and do ruku, finishing the rikaat and sitting for tashahhud and salam.

witr is part of isha prayers, wajib in hanafi madh-hab.
It is supposed to be the last prayer of night.
It can be performed until dawn begins(fajir time).