Is it possible to retract a vow to abandon backbiting and to give charity on behalf of those who were backbitten, which was vowed out of ignorance, and suffice with striving against oneself to abandon backbiting?

Whoever vows to obey Allah, it is obligatory for him to fulfill it. A vow contingent upon a condition becomes binding if the condition is met. It is not permissible to retract a vow of obedience; rather, it remains binding. Your vow is binding, and it is not permissible to retract it. You are obligated to give charity on behalf of whomever you backbite, every time you backbite someone.