What should be recited from the surahs of the Qur’an during the night vigil (qiyam al-layl)? Is it better to recite short surahs or long ones? Is it permissible to open the mushaf and read from it during prayer if the recitation is from long surahs?

There is no obligation to recite specific surahs after Al-Fatihah in prayer. Rather, a person may recite from the short surahs or the long ones during the night vigil (Qiyam al-Layl). It is sunnah to recite from the long surahs if possible, otherwise, whatever is easy for him.

One can recite several short surahs for those who do not memorize the long ones to attain great reward. He who performs (Qiyam) with ten verses will not be recorded among the heedless. He who performs (Qiyam) with one hundred verses will be recorded among the devout. And he who performs (Qiyam) with one thousand verses will be recorded among the Muqanțirīn (those who receive immense reward).

There is no harm in opening the Mus'haf and reading from it in prayer for the imam or the one praying alone, but it is better to recite from memory because it is more conducive to heart presence and brings more khushu' (devotion).