If it’s just a re-affirmation, then there’s no problem. As you state, one cannot re-affirm an invalidity. However, if they are actual vows, then they will be judged on their own merits – in this case, the practical effect is much like a convalidation of an invalid marriage.
Here’s the case I’m making, as an example:
2000 – John and Sally are married in the Catholic Church; according to form, no impediments, an exchange of consent. It seems to be a valid marriage. Unbeknownst to all but John, he really doesn’t intend fidelity, and plans on keeping up a clandestine relationship on the side.
2005 – John and Sally ‘renew’ their marriage vows. The actual vows are just a ‘re-affirmation’, though.
2007 – John stops cheating on Sally. (Yay!)
2010 – John and Sally go to ‘renew’ their vows again (gee, we just love these renewals on our anniversary! they’re really meaningful to us!

). However, this time, the priest that celebrates the vows asks for and gets an act of consent.
2011 – Sally finds out that John cheated on her for the first seven years of their marriage. Boo!
Due to the circumstances, Sally decides that she’s been living a charade – John never really
meant to be part of a Catholic marriage. After the divorce, she starts the nullity process. Now, the 2000 ceremony may be found to be invalid – after all, John’s consent was defective. The 2005 ceremony is immaterial – it has no practical effect. However, because of the 2010 “renewal” (which was actually a new act of consent, and therefore, a valid marriage), they are found to be validly married. Sorry, Sally – no annulment possible. And why? Solely because of the 2010 “renewal”.
LOL. It certainly
is a distinct and particular case I’m making, and definitely, doesn’t cover the majority of situations that people experience. But, when these situations pop up – and they do! – these are exactly the ones that folks use as a club against the Church. “Oh, the Church is sooo mean! Sally is completely innocent, and look how they’re forcing her to never be remarried! WWJD? What a horrible institution!”
Yet, due to the seeming injustice involved, and the fact that most people in this situation don’t even know what real effects their “renewal” actually has, it seems like something that bears discussion.
I’ve explained my point ad nauseum. I’ll stop now, since hopefully, you can see where I’m coming from by now…
No problems…