Between the facts that a: VGF is new so a disproportionate amount of members want to stay there
b: VGF is not sold out so there aren't a lot of members yet.
c: December is a busy time
d: Many DVC members who own points at VGF did it as an "add-on" to existing memberships so they probably don't have many points, meaning they have more points at different resorts and smaller amounts of VGF points, enough to book there every couple of years at the 11 month window but not enough to stay there all the time...
Between all of that, it would probably be easier to try to book a room at VGF direct from Disney itself, IF staying there is very important to you, and knowing right now that you've got it. Of course, it will also be much more expensive.
But this doesn't mean that the DVC resale places like David's aren't good at what they do or not trying to provide their services to you. As others have written, David's gets a supply of DVC points to rent out when DVC members offer them their points to rent. They're the go-between, the third party that facilitates the deals between renters and owners. If they don't have the supply, they can't meet your demand.
Now, the question I'd ask is - what's more important, getting a vacation in December or getting VGF in December? If you can't get VGF, is that a deal-breaker?
I don't know if you can plan ahead with David's but if you can, I'd suggest asking them to try to book VGF for you in May, once you're within the 7-month window and any DVC points can be used. You might get lucky. But you can also give them backup options and you might get those too. If it's most important for you to get a WDW trip in December using rented DVC points, I'm sure they'll be able to get you something - even if it's "only" OKW or SSR but very possible BLT, VWL, VAK, BCV or BWV, too - and the rooms will be nicer and the amenities better than much of anything else in a Value or Mod at a comparable or acceptable price.