When a couple makes a promise to the Lord with their vows...who is breaking it when adultery occurs?
You mean regular old marital infidelity? I would say the one who acts. The one who is faithful is a victim.
2380 Adultery refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations - even transient ones - they commit adultery. Christ condemns even adultery of mere desire. The sixth commandment and the New Testament forbid adultery absolutely. The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.