Vow Renewals are not technically second weddings, as no divorce or death has occurred in between. They are usually held to celebrate big anniversary milestones (20 years, 25 years) or after the couple has overcome a major obstacle in their marriage. Etiquette is slightly different between renewals and weddings.
Some couples see vow renewals as a chance to "re-do" their wedding as it may not have been what they dreamed of at an early age or felt disappointment in how the party side of the event turned out. This is what is referred to as a "PPD" or Pretty Princess Day. PPDs are generally frowned upon as they tend to breach proper and traditional etiquette. To learn more about a PPD, click the link below: