Trip planned except Saigon
 

This was the trip planned by agent

Bangkok 3 nites
Chiag Mai/ 3 nites
Hanoi 3 nites
Danag/Hoi An/Hue 3 nites
Cambodia Seam Reap 2 nites Is this enough time to see everything and should we miss Saigon

Two nights in Siem Reap is absolutely not enough! You need at least three days, not one and two bits, unless you're not interested in temples at all, in which case don't go.

How much time do you have altogether? And what does Danang/Hoi An/Hue mean?
Have you read any of the trip reports here?

I agree with Thursdays. You don't have enough time in SIem Reap. It takes three full days to see just to "major" temples.

It appears from your itinerary that you have a total of 14 nights on the ground in Asia. Really that's only enough time for 3 or an absolute maximum of 4 stops. It will take you most of a day to move from one place to another, so your current itinerary allows you very little time in each place. And adding in HCMC would assure that you glimpsed each place and experienced none of them

If it were me (and it isn't) I'd suggest
Bangkok 4 nights
Hanoi 3 nights
DaNang/Hoi An/Hue 3 nights
Siem Reap 4 nights

Still that's a pretty packed schedule. Asia is not like Europe.

Do you fly back to Bangkok to catch your flight home?

Danang/Hue/Hoi An 3 nights means you're traveling each day (unless you mean you're staying 3 nights in each). I would skip Danang, and if I only had 3 nights, I'd pick Hue or Hoi An, but not both.

Danang is the airport for Hoi An and the 2 cities are very close to eachother. So I would consider it 1 destination.
I agree with all the others that Angkor needs at least 3 days.
To see the main sightseeings 3 days in Bangkok are by far enough.
If you just have these 2 weeks its better to skip Saigon and maybe Hue or Danang/Hoi An.