Is expiation obligatory for one who vowed to wear the "isdal" (a loose, modest garment) upon success, but did not fulfill her vow?

The legitimate hijab is an obligation that is impermissible to abandon or postpone. Adherence to it is obligatory, even if no vow is associated with it. If it is established that wearing the hijab is an obligation by the core tenets of Islamic law, then a vow concerning it is not binding according to the majority of scholars; because it is tahsil حاصل (achieving what is already achieved). Therefore, nothing is necessitated by this vow, but it is obligatory to hasten to wear the hijab and to continue doing so.