We've never felt the need to renew our vows. We did the 'till death us do part' bit and 45 years together strikes me as enough evidence of our commitment. Renewing them would make me feel that they'd somehow been forgotten.
Having said that, I know of people who have done it. One man surprised his wife at the daughter's wedding. Another was family (we weren't invited, but their adult children, and their children were).
I can understand why some people would want to do it, going by the stories on here, though.
Anyway smart, but not as fancy as a wedding, for dress code I would think.