I'm not sure you've understood, but I'll try again:
-The character table in FFVII is fixed at 256 characters, some of these cannot be modified because they are expanded to things like characters names
-You can easily replace most of the characters in the table with whatever utf8 character you like, it doesn't matter whether it occupies 1, 2, 3, or 4 bytes. These will be used by touphscript when scanning text to map a character to FFVII's single byte encoding.
-The Japanese version of FFVII had multiple font tables, these were selected using a 2-byte sequence, the PC version may still retain this functionality
-You might be able to squeeze the Vietnamese alphabet, digits, and punctuation into the table by replacing the characters you don't need, the total number of user characters available in the table are about 210, otherwise you'll need multiple tables like Japanese
N.b.: ASCII is a 1-byte encoding, in fact it can be packed into 7 bits; utf8 is a variable width encoding, from 1-4 bytes, but this has nothing to do with editing the character table in touphscript.