Thailand forum

Thailand:
NO, you will not even be able to check-in for your flight to Thailand if you don't produce an onward-ticket

Vietnam:
You will need a Visa beforehand before even thinking of traveling to the country

As has been discussed many times on this site, the issue is not which passport you are carrying, etc. It comes down to whether or not the airlines will allow you to board the flight, since they carry the can IF the country doesn't allow you in for some visa-related reason.

As many have stated in these forums, people are often asked for proof when flying into Thailand from outside the SEA region. Generally, if flying within SEA, it hasn't been an issue. BUT... it's always a risk. The requirements are generally that you provide proof of departure within the visa period. Some airlines will ask to see that proof, and others won't.

When I'm in that situation and choose to take the risk, I always have a printed itinerary with me (just an outline of my proposed plans and dates) that will indicate that I'm leaving the country within the visa period. I've never used it as 'proof', and of course it only shows my intention, and isn't really proof. But it's better than nothing.

Read up on all the other threads for further info and discussion points.