Asia - Time frame for Vietnam/Cambodia trip

I'm trying to figure out how many days to plan for a proposed Vietnam/Cambodia trip. We're thinking we want to visit Hanoi, Halong Bay, Hoi An, Saigon and then to Siem Reap for Angkor. We were intially thinking 10-12 days not including the travel to and from Asia. Is this realistic? Any suggestions would be great.

Having been to all the places on this list, here's how I plot it out:
Hanoi, 2 days
Halong Bay, 2 days (including travel to/from and overnight on a boat or island)
fly Hanoi-Danang, bus/car to Hoi An, afternoon in Hoi An, 1 day
Hoi An, 2 days
return to Danang, fly Saigon, balance of day in Saigon, 1 day
Saigon, 2 days
fly Saigon-Siem Reap, balance in SR, 1 day
Siem Reap, 3 days

That's a bare minimum total of 14 days (excluding travel to/from the region), moving quickly and taking flights, and not having time to see things like Mekong Delta near Saigon, or Hue. So, in conclusion, I think you either need to pare down your list, or increase your allotted time. If you are not flexible on time, I'd choose from one of the following plans:
1) Vietnam only, understanding that even then your time is woefully inadequate even for the highlights.
2) Cambodia only, there's a lot more to this little country than Angkor (splendid though that is)
3) Combo plan with severe limits to Vietnam. In 10 days, you could comfortably do a linear Saigon-Mekong Delta (daytrip or overnight)-Phnom Penh (travelling overland)-Siem Reap. This would be cheaper than internal flying and distances are not long and on good highways.

It's quite realistic for me, but I don't linger and move quickly.

Just on the Angkor site portion, the classic itinerary is:
--day 1 arrive
--days 2, 3, 4 temples and maybe several hours puttering around the lake
--day 5 depart

The overall site is huge, not to speak of remote temples, and many tourists wuss out when it gets toasty in all that stone, so their main remembered sights are G&Ts by the pool.

I don't do it much anymore, but I confess to in the past cribbing from tour itineraries to help plan my trips. Here is one from Intrepid (http://www.intrepidtravel.com/tripnotes/VAK/2009-01-01) you could easily modify to work for you, as I did below -

Days 1-2 - Hanoi
Day 3 - Halong Bay (o/n)
Day 4 - Return to Hanoi/Flight to Hoi An
Days 5-6 - Hoi An/Flight to HCMC
Days 7-8 - Ho Chi Minh
Day 9 - Flight to Siem Reap/Angkor Wat sunset
Days 10-11-12 - Angkor Wat/Tonlie Sap

Travel takes at least three full days with an itinerary like this, and two days to arrive and depart the region. Like most, I would suggest either more time or a less ambitious schedule. But I predict you will have fun no matter what.

We are getting single entry visas, so we're thinking of an itinerary like this. We will have 14-17 days to play with, and haven't decided how much time in each city yet.

Fly into SGN.
Fly/train to Danang to visit Hoi An
Fly/train from Hoi An to Hanoi
Fly Hanoi Siem Reap
Fly Siem Reap to BKK or SIN to leave Asia.

I'm all for flying each domestic segment, but taking a train on one of the segments might be quite nice. Danang/Hoi An looks almost equidistant between Hanoi and Saigon. Any advice on which leg would be most scenic?
If we have to we'll save our scenic journey for the bus/car from Danang to Hoi An.

Thanks...