The line of cars to get back through MK main entrance to VWL could potentially be very long. Here's what I would suggest take the back way past Ft. Wilderness.
I can't give you directions from Discovery Cove but what you should do it get yourself to Bonnet Creek Parkway. You can pick up BC Parkway from Buena Vista Drive. Look for the signs that say Old Key West, Port Orleans Riverside & French Quarter resorts. All of those resorts are off Bonnet Creek Pkwy.
Follow BC Pkwy past Port Orleans French Quarter & Riverside (they'll be on your right). You will also pass Best Friends Pet Resort (on the left)
Follow BC Pkwy until it ends at a traffic light You'll see Golden Oaks straight ahead of you. You have no choice but to turn left or right here. Turn left onto Vista Blvd.
Follow Vista Blvd past Ft. Wilderness Campground (on the right). Vista Blvd ends/merges at World Drive just past the main entrance to MK. You'll have no choice but to merge right onto World Drive. At this point you should recognize where you are. Turn right at the next traffic light onto Timberline Drive & voila you're home at VWL & avoided all the traffic!
We take this route all the time. It's easy & saves time. We'll also be at VWL from 12/20-29. No place else I'd rather be over Christmas. Enjoy your trip!