I visited Halong Bay as part of a two+ week trip to Vietnam. The Halong Bay entry can be found here
My trip reports on Saigon, Mui Ne, and Hanoi from this trip can also be found there. I will soon post on Sapa, and at some point get around to my reports of Hoi An, Hue, Danang, and Nha Trang from last year.
Hope it's helpful.