This cruise (Galveston 4/6) was on the 20% discount list when I was on-board the Magic over Thanksgiving, and assuming it still is at that special offer for you next week, I'd book the specific cabin you want at the guaranteed 20% off rather than waiting for a possible other discount.
I booked March 1 Magic for 20% while onboard and compared it to what I expected the corresponding VGT to be (if it ended up being offered based on earlier similar cruises), and I only paid a small amount more and was able to select my specific cabin as opposed to taking whatever may have been assigned with a VGT offer, so it was worth it to me.