Is it prescribed to pray the two rak'ahs of Shafa' (even-numbered) immediately after the 'Isha prayer and delay the Witr (odd-numbered) until the end of the night, or is it better to pray Shafa' and Witr with the night vigil (Qiyam al-Layl)?

The two rak'ahs after 'Isha' are the regular Sunnah (Sunnah Ratibah). As for Witr, it can be one rak'ah or more with a single taslim (salutation) or two taslims. What precedes Witr is considered shaf' (even-numbered), based on the Prophet's (peace be upon him) saying: "Night prayers are two by two, and when you fear the coming of dawn, pray one rak'ah for Witr."