For a civil wedding ceremony, no vows are required beyond stating before a public official and at least one other witness that both parties take each other to be husband and wife. Religious wedding ceremonies may have their own requirements for specific vows.
Wedding vows are often an important part of the wedding ceremony. However, they have no legal significance. MarriageLaws.com points out that a couple is not denied a divorce because they vowed "till death do us part." Whether using traditional vows or writing personal vows to one another, the vows are simply a cultural part of today's wedding ceremonies.
If the wedding is a religious ceremony, however, vows may be required. For example, the Catholic church counts marriage as one of the Seven Rites, so the wording of the vows is almost always the same. It is a required part of the Rite of Marriage, and the vows symbolize not only promises to each other but promises to God. In a traditional Jewish wedding ceremony, the couple does not recite vows to one another but claim each other as they exchange rings.
If a couple chooses to write their own vows, they can be as simple or as expressive as the couple wishes. The vows are promises of commitment between the couple and should reflect what the couple wishes for themselves throughout the marriage.