Our visit to Vero Beach was over MLK weekend. Our room was an ocean view room overlooking the pool, pretty much the view you see in just about every picture of the VB pool.
They did have a lot of planned activities, although we're not the type to usually partake of them. Although probably not at the same high-gear level of summer, I would say the resort was far from "dead".
It's true that the ocean water was rather too cold for comfortable swimming, but daytime swims in the pool were fine. We did have a nice, relaxing day renting beach chairs and sitting on the beach.
The weather was very pleasant, perhaps we were lucky?
For us, VBR is a great place to relax and unwind, and we'd consider going back even in January. Those who like a higher-octane beach experience, like at Ft Lauderdale or South Beach, are likely to be disappointed.