We take God's name in vain, when we make rash and unlawful vows. It is a good vow, when a man binds himself to do that which the word binds him to; as, if he is sick, he vows if God restores him, he will live a more holy life. "I will pay you my vows which my lips have uttered when I was in trouble." Psalm 66:13, 14. But "such a vow should not be made, as is displeasing to God;" as to vow voluntary poverty, as friars; or to vow to live in nunneries. Jephthah's vow was rash and unlawful; he vowed to the Lord to sacrifice that which he met with next—and it was his own daughter! Judges 11:31. He did bad to make the vow, and worse to keep it; he became guilty of the breach of the third and sixth commandments.

~ J.C. RYLE
The Ten Commandments