If you would like to restart varnish you will first need to test the configuration to make sure the restart does not stop the service:

Note: When you run the following command it should dump the output of the config on the screen if it is successful. If it fails then it will produce an error.

varnishd -C -f /etc/varnish/default.vcl

If succesfull then run:

service varnish reload

or

service varnish restart