Must Surat Al-Baqarah, which was vowed to be recited in order to abandon a specific sin, be read in one sitting, or is it permissible to divide its recitation throughout the day, especially given the uncertainty surrounding the phrase "in one sitting" at the time of the vow?

This vow is a vow of contention and anger. What is obligatory regarding it is the choice between fulfilling the vowed act or offering an expiation for an oath (kaffarat yamin).

If you choose to fulfill the vowed act, then you must recite Surat Al-Baqarah a number of times equal to the number of times you committed the sin. It is not necessary to recite it in one sitting.

According to the Hanbali jurists, these vows overlap, and a single expiation suffices for them. This is a lenient approach that removes hardship, especially for someone who believes they are afflicted with obsessive whispers (waswasah).