My wife and I tried it in June of 2000. It is not a maintained trail. They give you a map. You leave the resort by going under the road A1A. You travel down a nice sidewalk going left towards downtown Vero Beach for about 1/2 to a mile. Then you take a right onto a graded but unpaved road and you follow it to the Inland Waterway and ride along ithis road with homes on it til you hit Rt 510. You turn right and travel along the highway for 1/2 to 3/4 of a mile and you are back at the resort. Be careful of traffic on this road. My memory for distances is probably not quite accurate. I think the whole loop took us about an hour at a liesurely pace. If you have kids and are as cautious as I was when mine were young - you might not want to do the part of the ride that is on 510. In my opinion Hilton Head is a much better resort for bike riding because there are dedicated bike paths and you can ride through the gated community of Palmetto Dunes (where the DVC Beach House is located) where speed limits are low. It is my opinion that bike rides at Vero are more suited to adults than children because you are riding on streets.
Hope this helps.
Your comment about children not being particularly safe riding bikes at Vero is right on the money - only I found out about that after my 6 year old received stiches in her chin...you would think that such a family resort that rents bicycles would have addressed this better...