Is it permissible to abandon this vow – which is performing 6 rak'ahs after Maghrib and 7 after Isha daily – and suffice with fasting 3 days as expiation? And is making a supererogatory prayer obligatory considered a sin? And are 6 rak'ahs after Maghrib from the Sunnah? And is it permissible to pray them as a vow even if they are not from the Sunnah?

A vow (Nathr) is not established by mere intention or inner monologue; rather, it requires verbal articulation. The phrase "I will pray" is considered a promise that is not obligatory to fulfill, and therefore, you are not bound to perform the mentioned rak'ahs. A vow is a commitment to an act of obedience that is not obligatory, and making a conditional vow is disliked (makrooh) but not sinful. As for performing six rak'ahs after Maghrib prayer, a very weak hadith (definition) has been narrated concerning it. There is no harm in performing them as a way of filling time with voluntary prayers, without believing them to be attributed to the Prophet, peace and blessings be upon him, or consistently adhering to a specific number.