But I've been eating a lot with a Vietnamese family and they say "con gà" or "con heo" to refer to the food, so I guess it is correct (or at least usual) to use "con" for the food too.
I read elsewhere that you use con gà for e.g. a roasted chicken. Whereas you use 'mon' (sp?) gà for a prepared meal where the chicken has been cut into small pieces.