Does expiation become obligatory for one who returns to committing a sin that was vowed to be abandoned, after having abandoned it in fulfillment of their vow, and is the vow dissolved by offering expiation?

It is not permissible to commit a sin. If you return to commit it, you are sinful, and sincere repentance is obligatory, and expiation is required for breaking the oath. The expiation is not repeated with the repetition of the sinful act, unless a phrase indicating repetition is uttered; otherwise, only one expiation is required.