To implement a translation to another mindmapper, you may want to know of the
following entry from the manual, there is an appendix section, C4, that provides some
information:

C.4 VYM file format
VYM maps usually have the suffix ".vym" and represent a compressed archive of data.
If you want to have a closer look into the data structure map called map name.vym,
just uncompress the map manually using
 $ unzip mapname.vym
This will create directories named images and flags in your current directory and also
the map itself, usually named map name.xml.The XML structure of VYM is pretty self
explaining, just have a look at mapname.xml.
This XML file can be loaded directly into VYM,it does not have to be compressed. If
you want to compress all the data yourself, use
 $ zip -r mapname.vym
to compress all data in your current directory.