To read files using old encoding, well, for multiple technical reasons the best solution is to convert them to Unicode before starting any modification.
For that matter I wrote an extension for OpenOffice.org called OvniConv
, which I then enhanced with the help of 2 Vietnamese friends (on a request from the MoST) and the result has been made available as a new project (a fork dedicated to this request) called B2UConverter
. I strongly recommend to install the latter one, since it's currently the most achieved work (even if there is still a few minor bugs, but without loss of data, only some data left unconverted sometime).