67:5 If a person makes vows in order to correct his character flaws, he is considered zealous and praiseworthy. Examples of this include if a glutton vowed not to eat meat for a certain length of time, or if a drunkard prohibited himself from drinking wine and other alcoholic beverages, or if a person who is vain about his appearance accepted upon himself the conditions of a Nazir. Vows such as these are a part of one’s service to G-d, regarding which the Sages said, “Vows are a fence for abstinence” (Avos 3:17). Nevertheless, one should not get in the habit of making such vows; it is preferable to overcome one’s urges without vows.
67:6 A vow does not take effect unless a person’s words agree with his intentions. If a person made a vow by mistake, his intentions not agreeing with what he said, or if he thought about making a vow but did not express it verbally, it is not a vow.