DVC isn't like a normal hotel, there really are not true down times. The point of a timeshare is to have close to 100% utilization at all times. Now obviously this doesn't happen by chance, which is why there are different point requirements for each day of the year based on assumed demand.
I think your best chance of getting a GV outside your home resort would be just after Labor day in September to just before Food & Wine starts. From there I would say in between New Years and maybe mid to close to end of January. Outside of that it would be pure luck.