@George *****
A bit strange you couldn't get a flight to Ho Chi Minh considering there are so many. Never really heard of flights to Saigon being completely full except during Tet (Vietnamese New Year).
It's always possible to leave Thailand on time and get to where you want to go.
Not saying you did anything wrong.
However, what I would have done is to leave Thailand, enter Cambodia (by air or land) then continued onto HCMC.
A fairly stress free and relatively quick way of getting to HCMC is to fly to Phnom Penh then catching a 6 hour bus to HCMC.
Another option is flying into Phnom Penh and catching a flight from there to HCMC.
Or what you did, fly to Hanoi, then catch a connecting flight to HCMC.
Or fly to Danang, then connect to HCMC.
Or fly to Vientiane, then connecting flight via Phnom Penh to HCMC (on Vietnam Airlines). The stop in Phnom Penh is a technical stop; you can remain on the aircraft during the layover.
Anyway, just saying there are lots of options and if I had been in your situation, I'd have gotten myself to HCMC rather than Hanoi knowing it's a simpler process down there.