I am travelling to Vietnam in December and would like to book with a travel agency in the Hanoi area. There are so many to choose from, what are things that I should consider when picking a travel agency (additionally to avoid getting ripped off).
My experience with Vietnam was that it really didn't matter - there are literally hundreds of travel agencies and many of them share the same tour operators. You get picked up at the travel agency you booked at and the bus continues on to 5-10 other places to pick up everyone else. The only thing you really need consider is the price (which shouldn't vary by much anyway).
If you're really concerned about getting ripped off (unlikely), I would wait until you arrive, and just ask some recently returned travelers who they booked with and whether they would recommend them. You may even get a (small) discount if you say you were referred by someone else.
For Ha Long Bay, most tour operators will recommend a 1-2 night tour, with one night on Cat Ba Island and one on a Vietnamese junk (tourist boat). For what it's worth, I thoroughly enjoyed both. Be warned that a junk recently sank during the night killing a few tourists, so be a bit more wary of spending a night on one.
Similarly with Sapa, tour operators will recommend you do some trekking with/to meet the local Hmong tribes, this I would also thoroughly recommend. Don't forget to bring some decent hiking boots (or buy them in Hanoi) as it's always muddy. After the hike you can get your boots cleaned and dried in 20 minutes for about a dollar. I have no idea how they do it.
Tours are very easy to organise from Hanoi but keep in mind if you're going far afield it can take a very long time due to the condition of the roads and the fact that speeds are limited due to motorbikes/pigs/chickens/people sharing them with you. This shouldn't be a concern with what you're interested in, but something to keep in mind if plans change.
I agree with Jozzas. It doesn't really matter.
I recently returned from Vietnam. Most hotels that you stay in at Hanoi have tie-ups with some tour operators, especially for tours to Sapa and Ha Long Bay. I did not travel to Sapa, but we had our hotel book us a 1 night-2 day Ha Long Bay tour. We travelled with Cristina Cruises on their Vietnamese junk boats. It was a wonderful experience, and we had a good time. We did not go to Cat Ba Island but were taken to one of the biggest Cave islands in the Bay. But most cruises offer a variety of options that you can pick from.
My recommendation: check with the hotel you are staying with if they have any tie-ups with tour operators, and go with their suggestion. There are many tour operators in Hanoi (if you walk through old market streets you are likely to be confused with all the tour offers). I would not recommend more than a 1 night - 2 day cruise on Ha Long bay, though.
For Ha Long bay from Hanoi you can just get yourself a bus ticket from the bus station. If you are considering Ha Long Bay also consider Cat Ba Island. I have heard mixed reports of Ha Long Bay tours. I was warned a really cheap tour is generally a really bad tour but I rock climbed with a guy on Cat Ba Island who took a cheap tour and said it was great. I think the quality of the sleeping on the boat was generaly the issue. I travelled by bus to Cat Ba with Hoang Long Bus Company. I think they go to Ha Long city as well. If you buy a Ha Long Bay tour though, it may include transport from Hanoi.
If you don't have a lot of time - like one week or so - you should book a tour online. But if you have around one month come to VN and take the tour from hotel or local agency!
Halong Bay - (should take packet tour)- check weather forecast before booking the tour
Sapa - if you want freedom time just book transfer to there and take tour or do it yourself hen you be there that is easy way!
How to get south:
Hue - one day (from Hue to Hoi An). If you are not scared of heights I recommend motorbike way. That is imagine way!
Danang - Hoi An two days - three days! Visit my sons temple - Cu Lao Cham Island - Hoi An (the best place to make tailor like suite, dress, shoes, etc).
Nha Trang - or Mui Ne for relaxing beach
Ho Chi Minh - one or two days - Mekong Detail - Cu Chi Tunnel
Phu Quoc Island is a beautiful beach.
You can get more detail when you arrive.
Have a good trip!