We will be in the exact same boat in August. Three days at Vero then four at WDW. I'm assuming that on our transfer day it will allow us to buy G+ because we have a WDW DVC resort booked for that night, plus a park pass and so on. It's no different than any first arrival day at Walt Disney World. I know in March on our last visit I was allowed to book G+ and ILL from the airport at home because we were checking in on that day. However, it's possible this is different now since you cannot pre-buy G+ any more. We will have to see, but I'm certainly hoping and assuming we are good to purchase G+ on that day with a valid WDW DVC resort res for that night and theme parks tickets.
We will see if anyone has any practical experience on whether or not they were prohibited to purchasing G+ on their WDW check in day...