Siem Reap, Phnom Penh and Ho Chi Minh

Hi this is our dilemma.

We have read most of the posts with great interest as we would like to do a trip. This is our fourth trip to Pattaya and we think it is now time to visit Cambodia and Vietnam.
Our dilemma is mini bus or plane and which way round to do the trip. We need to end up back in Pattaya.
Any suggestions would be appreciated. Also, is there a beach resort we could visit in Vietnam or any other must do places to visit? The good news is we have time to plan our trip as we are in Pattaya now and will be here for a while.

Thanks, Barney & Betty