I don't believe the transit visa is available for individuals, but rather to those on tours or cruises. Airlines will not board you without proper documentation for Vietnam. I don't believe Christmas day is a legal holiday in any Asian country, but it will be crowded with westerners at that time. Christmas Eve in Saigon is still celebrated with special events and is very festive.

At least Tan Son Nhat HCMC Airport is close-in - book a hotel and have them pick you up (you have to be careful with cabs in VN).