dired-vm-read-only-folders

dired-vm-read-only-folders is a variable defined in `dired-x.el'.
Its value is

nil

Documentation:
If non-nil, M-x dired-vm will visit all folders read-only.
If neither nil nor t, e.g. the symbol `if-file-read-only', only
files not writable by you are visited read-only.

You can customize this variable.