Is a covenant with God to perform supererogatory prayers considered a vow, and what is incumbent upon one who makes such a covenant and then falls short in fulfilling it?

We congratulate the questioner on being guided [to Islam] and ask Allah to grant him steadfastness. We have previously explained the scholarly difference of opinion regarding the covenant of Allah. The preponderant view is that it is considered an oath, based on Allah's saying: "And fulfill the covenant of Allah when you have covenanted, and do not break the oaths after their confirmation." Therefore, it is obligatory to fulfill the covenant. If there is a shortcoming in fulfilling it, an expiation for an oath becomes binding: feeding ten needy persons or clothing them, or freeing a slave. If one cannot find [any of these], then fasting for three days.