December is monsooon season in the north
You are going to do all of VN in a week? Hanoi to Saigon with stops in Ha Long Bay, Hue, Phu Bai, Khe Sanh, Danang, Chu Lai, Nha Trang, etc., it and it took us a month. You could spend 2 days walking around Hanoi, a day in Saigon and a day at Ha Long Bay alone. DO NOT skip Hoi Anh - it's great at night and very nice in the day.
In December, expect monsoons from Qui Nhon north to Hanoi - many times you may find the roads closed and airlines diverted. Last NOV-DEC we were going to drive from Hanoi to the DMZ and down to Danang, but the roads were washed out and the airlines couldn't make it to Hue because of bad weather. We had to fly to Danang and resume the driving part of the trip from there. We ended up driving in heavy rain north from Danang to visit Hue, Phu Bai, Khe Sanh, DMZ, etc.
After heading south out of Danang for Saigon, we finally got out of the rains just north of Nha Trang. That's the same weather pattern I experienced when I was flying there in '69, so I wasn't too surprised, but it was also the only time I could get away for VN last year.
Why ride in all that Saigon traffic? That's kind of nuts - you won't see anything but 3 million scooters and lots of cars and trucks. Get a hotel near a massage parlor (hard to avoid the massages) and walk the city, use buses or pedi-cabs or taxis and treat yourself to a foot massage (or more) every night for about $10-12.