Have you considered going on your own? While we used Tonkin for train tickets to sapa and they were very reliable, travel in Vietnam was easy without a tour. If you click on my name and email me, I will be happy to share my extensive notes. You can also read my trip report if you haven't already done so.

I'm currently on a private trip booked through Tonkin. Don't know why you're having problems. Try contacting them using tonkintravel@fpt.vn. They have been easy to work with.

Thank you both so much! That is a different email address than the one on their website, so I will try it. Marija, are you happy with your trip so far? Anything I should know in working with them?

Although we are independent travelers, we used Tonkin a few years ago and were very glad we did. We had a flight cancelled due to weather. Tonkin rebooked and got out our hotel reservations cancelled at no fee (after the cancellation time). They were easy to work with and flexible.

I have used them too, they booked flights and some hotels, the rest I did independently. I was VERY happy with their service.

We used Vietnamstay.com, aka aurora travel and they were courteous, quick and efficient. They got better prices (by a little bit) than we could get on hotels. We didn't take any tours.

Tonkin delivered as promised, no problems. They also take credit cards.

There are many other local agents such as Asiatica, Buffalo, Exotisimo. They have biggest shares of customized tours in Vietnam.

We returned on Feb. 12 after 35 days in SE Asia including 23 in VN. We used Tonkin Travel for all of the VN part of the trip and also for flight tickets in Laos and Cambodia. I just reviewed my email correspondence and found the following address: hang@tonkintravel.com, as well as the one identified by Marija above We are independent travelers but as we get older we appreciate a bit of ease and comfort. Tonkin Travel was completely dependable, and met every commitment promised in the program. I recommend them highly and will use them again the next time we travel in that area.

We used Tonkin for four weeks in Vietnam in 2011 and they were mostly pretty/very good for local tours, airline tickets, car and driver for sightseeing and local tours, and airport transfers.
I especially liked the planning flexibility in asking for different hotels than the original ones quoted, juggling dates, and pricing different options. In one or two cases where I could do better on my own, they either met the price I found or allowed a DIY option.
You can ask for quotes from 2 or 3 agencies and see which one does the best for you.
My only complaint about Tonkin was being left unaided due to a weather-related cancellation of the Halong Bay part of the trip. We were pretty much left on our own to find an alternative Hanoi hotel for the additional 2 nights we needed to stay in the city.
I would add that some of their city guides were superb, in fact in my experience better than either Tong of Bangkok or Dara of Angkor Wat (both highly recommended on this board).