Vow renewals seem like the new thing to do. I had 8 weddings last summer and it cost me a fortune. I love every one of those friends and wish them well however, if in 5 or 10 years, I have to do the whole wedding circuit again via vow renewals, I might pull my hair out and it's not because I don't want to celebrate love happiness and commitment- it's that I thought we already did that.
Personally, I would only do a vow renewal if I went through a traumatic life event or our relationship went through some REALLY trying times. It's not like vows expire- they are supposed to be life long. If you really just want an excuse to wear a fancy dress again and have a big party, why not just have a fancy 10th anniversary party and wear a white dress? Or have a private, just the two of you, vow exchange while on a romantic vacation? I already gave my friends nice hefty cash gifts, gave up long weekends, celebrated their love, and support them planning a life together. Why do it all over again?