What great sin does a person commit who deliberately calls on God to bear witness to a lie? A person who deliberately calls on God to bear witness to a lie commits the very grievous sin of perjury.
Thou shalt not swear falsely by my name, nor profane the name of thy God. (Leviticus 19:12)
What is a vow? A vow is a deliberate promise made to God by which a person binds himself under pain of sin to do something that is especially pleasing to God.
When thou hast made a vow to the Lord thy God, thou shalt not delay to pay it; because the Lord thy God will require it. (Deuteronomy 23:21)
Further reading: CCC 2101-2104