Answered by Shaykh Yusuf Weltch
Must I fulfill a vow that I made to Allah if I am unsure whether I pronounced it verbally or not?
In the Name of Allah, the Most Merciful and Compassionate.
You are not obliged to fulfill the vow; however, it is recommended to do so anyway out of caution.
Back to the Basis
Whenever there is a doubt in a matter, the Sacred Law guides us to refer things back to their basis. Regarding things that must be pronounced aloud, the basis is that you did not verbalize the vow, and establishing the contrary requires proof.
Caution Is Recommended
Despite the above, the Messenger of Allah (Allah bless him and give him peace) said, “Leave that which causes you doubt for that which does not.” [Ahmad]
Due to this narration from the Messenger (Allah bless him and give him peace), caution is recommended.
Allah knows best
[Shaykh] Yusuf Weltch