We had 19 nights on the ground for our trip in March 2011. We broke it down as follows, which turned out great:

Hanoi - 5 nights, including Halong Bay -- we did a day trip, which was a mistake. I think your 5 nights including an overnight in Halong Bay is the right way to go.

Hue - 2 nights (this allowed 1-1/2 days of sightseeing, which was "just right").

HoiAn - 3 nights (this allowed 2-1/2 days of R&R, which was great. We were supposed to have 3+, but on the third morning, the weather was very threatening, so we opted to leave early for Saigon..., and then left Saigon early for Siem Reap)

HCMC - 2 nights (this allowed for 1-1/2 days of sightseeing, again, "just right". We did not go to the Cu Chi Tunnels or the Mekong Delta, neither of which interested us. If you want to do the Cu Chi Tunnels, you'll need at least 1/2 day, and I guess the Delta would require an overnight).

Siem Reap - 4 nights (we had 3-1/2 full days, of which only 2 were spent "templing" (we are not temple junkies either, I always say we're "temple lite"...want the highlights, not the PhD). The first 1/2 day we spent around town, and the last day, we spent at the pool).

Hong Kong - 4 nights (3-1/2 days...could have used at least another day, but it was a great introduction).

We also had never been to Asia before, this was our 60th birthday present to ourselves. We are very "independent" travelers, and definitely not "staid".

Four nights would be better than three for Siem Reap. It sounds like you might enjoy taking a boat from Siem Reap to Saigon. I think there are now several luxury cruises available. The budget option is a bus to PP and a day boat ride from there.

Remember that three nights is just two full days. I'd vote for 4 nights in each Siem Reap and Luang Prabang at a minimum. If you want to fly home from somewhere else, I'd suggest Bangkok. It's my favorite city in the world. Lots to do and see, and a great gateway city. I'd also suggest adding a couple more days so you can have 4 nights in Bangkok.

i love what sf did.. from your plan i would want at least 1 more night at siem reap...
i could add another night to hanoi---remember you loose almost 1 day going each way to halong.

remainder spend in bkk or spread them out and fly home from HCMC