Is it obligatory to fulfill a vow of seeking forgiveness along with the "bāqiyāt as-sālihāt" (the enduring good deeds) when in doubt about it after forgetting the vow and then remembering it?

As long as you are unsure whether you made a vow to perform the Everlasting Good Deeds (al-Baqiyat al-Salihat), the fundamental principle is that you did not make such a vow. This is because a vow is not established by mere doubt. Therefore, nothing is binding upon you. We warn you against succumbing to obsessive thoughts (waswas).