Must one fulfill a vow to refrain from sin that was made under duress and that the one who made the vow was unable to uphold?

You must abandon the sin and repent from it, whether you vowed to forsake it or not.

As for the validity of the vow, a vow to abandon a sin is considered a vow of an obligation, which, according to the majority of scholars, is not valid, and no expiation for breaking an oath is required if it is not fulfilled.

The phrase "I will stop doing something" is a matter of dispute among jurists; some believe that it is not considered a vow unless explicitly stated with "For Allah is upon me" or similar wording, while others believe that circumstantial evidence is sufficient for its validity if it indicates an intention to commit.