IMO itinerary #3 is the best of the three, but should be adjusted. With so few days, you really do not have time to go to Siem Reap. You would need at least 3 days if not more to enjoy the sites there so it is best left for another trip.

Hanoi deserves at least 3 days. I would also add a day to Sapa, trying to get there or leave from their on a Sunday so that you can visit the incredible Ba hac market either upon arrival or on the day of departure.

Perhaps you can go straight to the airport from Halong Bay, flying to Hoi An (Danang)or into Hue.

Hoi An is magical and picturesque, but can be seen in a day. We enjoyed wandering around for two days and spent a third at the beach. If your goal is to have clothes made, then you will need the three days, however, if you want to soak up the atmosphere, a day or two is fine. I would try to get to Hue if possible. If you leave Hoi An early in the morning, you can get to Hue in time to visit the Citadel and the following morning you can visit the Pagoda and Emperors' tombs (probably 3 should be good). There is an airport in Hue and likely flights to Saigon from there. While you would have to curtail your time in Hoi An, IMO Hue is well worth the time and perhaps even more interesting than Hoi An. Of all the places we went to in VN, we liked Saigon the least, although if possible, you should spend 1 1/2-2 days there.

You do not have enough time to visit Angkor. It takes three full days just to visit the major temples. One day would be an expensive and unsatisfying side trip. Spend the time you have in VN.

agree with the above comment. Not enough time for Angkor. Just take your time in Vietnam, a lot to see. But I prefer 3 days 2 nights in Hanoi and 2 days 1 night in HCMC.

Do Itinerary #2, but forget Angkor Wat this time. Add a bit more to Hanoi and maybe eliminate Nha Trang.

Vietnam is very diverse and we found that the very different images of different places needed time to sink in and appreciate. Half the fun was lingering in markets, restaurants and at food stalls and watching life go by in the different places rather than necessarily seeing all the sights in one fell swoop. I therefore agree with other posters that trying to cover Siem Reap as well is a step too far and best left for a future trip.

I'd also second Dgunbug's suggestion that you seriously consider Hue [which you can take in just before or after Hoi An] and maybe miss Nha Trang. We liked Hoi An a lot but be aware that it is nowadays a very tourist orientated town.

So we just returned and I promise a trip report shortly. Are you absolutely set on going to Sapa? We did not because by the time you do the overnight train, a day or two there and the train back you have spent a lot of time. I agree that if you are having clothes made in Hoi An you need at least two full days and three is better. I had a suit made and it is great but spent a lot of time up and back to Yally for fittings. Regarding clothes I would definitely do it in Hoi An because later in the trip I tried in Bangkok and could not find the fabric selections I wanted as the tailors are more geared to men. Also if having clothes made either take sample or pictures because there is definitely a language barrier which makes it hard and they give you tons of books to look through.

We stayed one night in Halong Bay but again it is a 4 hour ride from Hanoi and 4 back. I would not have wished to miss Angkor Wat. After spending a morning and evening there and also at Angkor Thom I had enough of it. But being so close unless you plan to go back it was a highlight for us.

We arrived at Halong Bay in the afternoon, toured the caves, skipped the kayaking. It was very cold so did not sit on the deck much. Left early next morning in time to have another 1/2 day in Hanoi.

Having been to sapa, I would say it was a highlight of our trip to VN and well worth the trouble of the overnight trains. I would go to sapa before taking the time to go to halong bay. While we loved seeing the bay, the ride to and from there was long and boring, taking up a great deal of time on each end. Traveling by train to sapa, you do not miss any tour time as you leave late after a full day's activities and you arrive early enough to have a full day in the sapa region.

I feel the opposite of dgunbug -- I loved our 2 days/1nt on Halong Bay -- it was foggy but still beautiful. Sapa was ok -- perhaps because we spent 4 days driving thru the countryside and staying in some small villages on the way to Sapa, by the time we got there I felt we had seen so much like it. And I did not like the train ride back from Sapa to Hanoi. But as dgunbug said, you don't loose any time since it is an overnight train. We went from the train station to the airport and flew early am to Hoi An which worked well.
Agree with those that suggest Hue as it was a very interesting town. Also agree to eliminate SR.

I concur that halong bay is worth visiting despite the dreadfully boring ride to and from there. I merely wanted to point out the convenience of the overnight train. Granted though, it is not the smoothest of train tracks and it can get rather smokey if the nearby compartment is filled with smokers. This happened to be a bit of a problem for my husband who had an asthmatic reaction. The cooler air in sapa was delightful after spending some time in the heat of Vietnam.

Yestravel - I seem to remember reading your report about your interesting experience with a car and driver taking you to sapa and I was envious that we hadn't done the same. Unfortunately, the OP doesn't have the time for that kind of exploration. Just curious, do you remember how much that cost you?

@dgunbug -- We had Tonkin arrange much of the 3 week VN portion of our trip to SEAsia. We paid for a driver and a guide. Hotels and food were very inexpensive. As we were planning the trip Tonkin did break out the costs, but at this point I really don't recall what we paid.

i would want more time in hanoi...

i like 3 but without angkor---one day is almost a waste given the expense and time to get there..

i also would skip either halong or sapa and redistribut the days and maybe add hue..