وكذلك لو نذر بالصلاة في الأوقات المكروهة يصح؛ لأنه نذر بعبادة مشروعة
A vow to pray at a disapproved time is also analyzed by distinguishing prayer, a recognized worship, from the disapproved time selected for it.
The prayer can be delayed until an allowable time, so the obligation of the vow can be fulfilled without performing it at the prohibited moment.
The key is that time as a container can be distinguished from the essence of prayer. This feature produces a different analysis from fasting on a specifically prohibited day.