dired-bind-vm is a variable defined in `dired-x.el'.
Its value is

Non-nil means "V" runs `dired-vm', otherwise "V" runs `dired-rmail'.
RMAIL files in the old Babyl format (used before before Emacs 23.1)
contain "-*- rmail -*-" at the top, so `dired-find-file'
will run `rmail' on these files. New RMAIL files use the standard
mbox format, and so cannot be distinguished in this way.

You can customize this variable.