How can a worshiper guarantee the reward of a full night's worship when performing Tarawih and Tahajjud prayers, given the differing number of Witr rak'ahs between the two prayers?

Tarawih prayer concludes with Witr prayer led by the Imam. Whoever prays with the Imam until he departs will have the reward of praying the entire night. This is achieved by observing the first congregational prayer after 'Isha'. If one wishes to perform another prayer in the latter part of the night, they should not perform Witr twice. They can either add a rak'ah to their first Witr prayer to make it even, and then perform Witr at the end of the night, or they can suffice with the first Witr prayer and pray with the second Imam at the end of the night without performing Witr, or add a rak'ah to the second Imam's Witr to count it as part of the night vigil. It has been narrated that the Prophet, peace and blessings be upon him, prayed after Witr.