I live about 25 minutes north of Vero Beach resort on the same barrier island. The weather in January can be really variable! I would count on needing a jacket in the evening, but you could very well be on the beach or in the pool in Jan.
I do have to mention that we have had really bad red tide as of late. It seems to have dissipated some this week, but it keeps coming and going. I know that it was bad as far south as Sebastian, so I'd try to find out if it's been affecting VBR before booking a trip there to enjoy the beach. Last weekend we couldn't even go outside w/o coughing terribly. It's been really hurting the beachside businesses because everyone is avoiding the beach right now.