Is a person considered negligent if they vowed to spend a night in prayer by reciting the entire Qur'an but were unable to complete its recitation that same night, instead finishing it on other nights, and what must they do?

If you are unable to fulfill your vow (to recite the entire Qur'an in one night during prayer), you must offer an expiation for breaking an oath (kaffarat yameen). However, if you are able to fulfill it in the future, then you are obliged to do so, as long as you did not specify a particular night for fulfilling the vow. It is permissible to complete the recitation of the Qur'an in less than three nights; this depends on the reciter's energy and circumstances, as there is no specific number of days prescribed for its completion.