PBI has its own on-ramp and off-ramp for I-95, so it is really very easy. It is a bit more than 90 minutes, and directions are easy: Rent car, leave PBI garage, take I-95 on-ramp, drive to Sebastian exit, east on 512, right on 510, after high bridge, right on A1A, left into Disney's Vero Beach Resort, enjopy a "Welcome Home" as you pass thru security, park, check-in, enjoy your vacation...
Melbourne is closer geographically, and there is said to be a shuttle van that will take you to VB, but we have not taken it. MLB is even smaller, and generally more expensive for us, plus the time savings on the drive is just not that significant (not enough to trade a non-stop flight for a connector, at least).
Actually, we find MCO to be easier than either PBI or MLB, but that's just bcs we are travelling with DS in his wheelchair; we just find MCO to be a very wheelchair friendly airport, and tohers to be less so. We also like that we can walk to the car rental place from within the terminal, rather than riding a shuttle in PBI. In our case, we are also better able to get SkyCap assistance in MCO, since we're not competing with cranky snowbirds at PBI.
Good luck!