My daughter and I have booked flights to these places, 6 nights in Siem Reap. 2 nights Hanoi and 4 nights in Saigon, We have been told that its too long to stay6 in SR, any suggestions for a good itinery, Thank you, Anne

Unless you are really into temples then 4 days is about right to cover the major ones and perhaps a visit to a floating village out of town. Forget the one most tourists go to (Chong Khneas). Then you could take a bus or hire a taxi to Phnom Penh and spend 2 days there visiting the sights. Fly from here to either Hanoi or Saigon if you can change our already booked flights.

I think four days would be about right in Siem Reap. It's a small place and it would be enough to allow a couple of days seeing temples and a couple more doing other things like shopping, visiting the cultural centre or looking around the market or the town in general.
We found seeing two temples was quite enough - very hot and dusty, also tiring.
If you cut back on Siem Reap, you could spend the extra 2 days in either Hanoi or Saigon (we were in Saigon at Easter and thought it was great)
We didn't get to do all we wanted in the time we were there but enjoyed it just the same - see my JBR.