Thank you everyone.

I had a 15 digit China Airlines code that ended in 01 indeed. And my name was also mismatched (China Airlines booking site did not accept two first names, so I only entered my first first-name). I tried entering the first 13 digits of my ticket number, and was presented with the manual upload form.

As for Vietnam Airlines, it does indeed just look like their system is down.
Checking my eb-retro request, the json response includes:
"errorCodes": [{
"errorCode": "200",
"errorMessage": "COULD_NOT_CONTACT_TICKETDB"
}]

Does that mean it used to work, and is down temporarily and I should just try again in a few days?

Thank you in advance