How to copy file from a Vagrant machine to localhost

Another option is cat the files to something local:

vagrant ssh -c "sudo cat /home/vagrant/devstack/local.conf" > local.conf

This should also work for files that require root permissions (something the vagrant SCP plugin doesn't seem to support).

You should read the manual page for scp. The correct syntax is:

scp -P 2222 [email protected]:/home/vagrant/devstack/local.conf .

The uppercase P is for "port". Lowercase is used to preserve modification times.