Some comments:

* Please provide some background information how widely the encoding is used. I get less than 1000 hits in Google when looking for "TCVN 5712:1993". Now, the encoding was a standard in Vietnam, but it has been updated in 1999 to TCVN 5712:1999. There's also an encoding called VSCII.

* In the file you write "kind of TCVN 5712:1993 VN3 with CP1252 additions". This won't work, since we can only accept codecs which are based on set standards. It would be better to provide a link to an official Unicode character set mapping table and then use the gencodec.py script on this table.

* For Vietnamese, Python already provides cp1258 - how much is this encoding used in comparison to e.g. TCVN 5712:1993 ?

Resources:

 * Vietnamese encodings: http://www.panl10n.net/english/outputs/Survey/Vietnamese.pdf

 * East Asian encodings: http://www.unicode.org/iuc/iuc15/tb1/slides.pdf