I guess my next question should be, "Do you speak Vietnamese?"
Hopefully you have other lenses and can still get some good shots and enjoyment from your travels. Terrible time for a fail.
But, since you are welcoming guesses, sounds like a bad AF motor and/or gear. I've traveled in Vietnam. If you can walk in to a true Canon service center, sounds good--if you have the time to wait. But nobody can tell you how long or how much except the techs at the service center you go to. I understand the anxiety and waiting until Monday, though.
But, wow, finding somebody who can tell you about a repair in Vietnam, based on your description...Even if a US CPS tech were here to chime in, that's a bit of a stretch.
If you have ever used CPS, you know the price could be pretty much anything they decide, and then it might change upwards.
Hope the rest of your trip goes better.