All these exceptions are manually added here, one by one, as you can see. In order to add your own exceptions you can follow these steps.
1. Understand how to map the characters. EG: ộ will be mapped to o
2. Go to http://www.ltg.ed.ac.uk/~richard/utf-8.cgi
3. Input the character ộ and select the option Character in the Interpret as radio button and click Go.
4. In the result table note the values in Hex UTF-8 bytes, in this case they are E1 BB 99
5. Edit the qa-include/util/string.php file and add, immediately below this line, the following code:
"\xAA\xBB\xCC" => 'D',
Replacing AA with the first pair of values from the page, BB with the second and CC with the third one. Replace D with your replacement character. In this particular case it would look like:
"\xE1\xBB\x99" => 'o',
6. Repeat steps 1 to 5 until all exceptions are added. Note that the length of values returned from the page might be different than 3. In that case, just use less separators (removing the additional \x, as in the file itself)
7. Navigate to admin/viewing and enable the Remove accents from question URLs setting
8. Enjoy and buy me a coffee :)