The older documentation in README.mysql says that "if you want" to convert cdb domains to mysql you should use vconvert; so I think that at least cdb and mysql can be used at the same time. Is it right or wrong?
I haven't figured out how vconvert does it yet. I will have to one of these days, but not soon. I still run a straight cdb shop.
My guess is that vconvert actually does cdb within itself, and provides one SQL back end, the one you selected at compile time. Even though the program --help seems to imply that you can vconvert from mysql to pgsql I don't think it is possible. I would love it if someone who knows would explain it...
I recently converted from cdb to mysql. I just used the vconvert program and it just converted it.
vconvert -c -m
Works like a charm for me.