Time of year and the number of units have a big effect on how difficult they are to get. There are only 6 beach houses at Vero, so the competition is stiff. BWV also has a limited number of GVs. OKW offers quite a few so, if it's not a holiday time, you have a better chance of booking one of these. I think SSR will also have a fair amount of GVs.
If the GV trip will be an occasional thing, calling right at the 7 month mark and maybe spending some time on the wait list could produce good results. (You could also consider finding a member to trade with.) If you plan on spending every Christmas in GV, you would be wise to purchase at the resort to give you the 11 month window.