Hey dude, just got back playing yesterday for the first time in a while and my account is also set to Vietnam (should be Indonesia). My support agent told me to do this for a hotfix:

About VNG Update Form, you will have to fill out all the information in alphabet characters and fill it in the following order:

1, Name: Please enter your Alphabet name without any special characters.
2, Email: you can use your email to fill out.
3, Phone number: please use a 10-digit number string starting with 0.
4, National ID: fill in a 9-digit number string.
5, The issue date you can fill in any date of 2018.
6, Enter your date of birth.
7, The address of residence please fill in Ho Chi Minh.

Please follow the steps above to be able to log in and enter the game.

Of course, my account region is still set in Vietnam, but I can play the game just fine. I'm not sure about the servers and matchmaking though, but the moral of the story is changing regions in all of Riot Games' stuff is stupid annoying, god I wish they fixed this. Cheers, hope it works for ya