I have lived in south Florida for several years. We have had 80's and we have had frost. It is cooler in Vero, as it is about 2 hours north of me. It is usually nice in mid-late December, 50's-60's, but it can be hot or freezing.
I know this doesn't give you a firm answer, but in the winter the weather can be a little flukey. (Is that a word?) It is best to be prepared.
And, on the bright side, it is always warmer than Oshkosh, and other northern cities, here in the winter!!
By the way, we love VB, and go as often as we can!!!!
iluvtig