In this case, I'd cut Thailand entirely and make it Vietnam + Cambodia only. Saigon would be the logical other "major metropolitan city" added to a HAN + REP mix. If you want some beach time, I'd just do Mui Ne which is accessible from Saigon by road.

Option A: No need to go to SIN:
--HKG-HAN (3 nights)
--internal flight HAN-SGN (2 nights)
--to Mui Ne (3 nights)
--return to SGN, fly to REP (VN, expensive) (3-4 nights)
--road to PNH and fly out on aft flight (or overnight if next-day morning flight)

Don't route yourself on an itinerary that takes you back to Vietnam for your homeward bound, unless you can transit without having to get another Vietnamese visa.

Hanoi itself only needs 2 days, add time if you plan on going to Halong Bay for the overnight stay-on-board cruise.

If you dispense with the beach bit, then I'd put an intermediate stop between HAN and SGN--probably Hoi An for 2-3 nights (airport is DAD-Danang), then fly on to SGN and pick things up.

If the SGN-REP nonstop flight pricing is beyond what you'd wish to pay, then do the overland via PP but it will eat up the entire day traveling.

Besides road between PNH and REP, there is also a flight option (I don't recommend as I have safety concerns and especially if rainy weather), there is also a boat option via the Tonle Sap...but research this first as to pros and cons. Some people have safety issues with this as well.


Option B: No need to go to SIN. Cambodia start.
--HKG-PNH (1 nt)
--overland morning to Siem Reap (3 nights)
--fly REP-SGN (expensive) or REP-HAN (usually even worse)
--internal flight between SGN-HAN or vv.
--exit via whichever you end up--SGN or HAN

Option C: Must fly out of SIN:
Similar to option A above but use the Silk Air REP-SIN nonstop flight, daily but the time it leaves REP changes dep on day. There's a Jetstar flight only on some days of the week, but it leaves early in the morning.

My inclination, award availability permitting, is to start this trip in Hanoi, as weather there becomes iffier (rainier and more prone to flooding) the closer you get to November. The more southerly areas SGN and Cambodia shouldn't be a problem.