Regarding PHL-LAS on 12/19, how flexible are you regarding flight times, and are you aiming for a non-stop? There may be seats that day but not necessarily on the most desirable fights.

I'd put low odds on finding SEA-PHL award seats on Sunday 12/28, and high odds on the last flight of the day being full but with no VDBs required. Others with more experience may have a better crystal ball, though.