Does the inability to fulfill the promise of "intervening upon seeing a young man harassing a girl" necessitate an expiation for a vow, knowing that the intention was for obligation and not for a vow?

Your statement, "A vow upon me," is neither a nadhr (vow) nor an yameen (oath), so there is no expiation for it, as the conditions for the formulation of either are not met. However, it is obligatory to condemn sexual harassment and protect the honor of chaste women, and this obligation is reinforced by one's commitment to it. Harassment is forbidden and reprehensible, and it must be repelled by the Sharia-compliant regulations for enjoining good and forbidding evil, because complacency in protecting honor might be a form of Satan's intimidation.