Add the extension to your CKAN .ini file:
ckan.plugins = ... vocabulary_services ...
Create the database tables:
ckan -c path/to/ckan.ini vocabulary-services-init-db
This will create the following two tables:
vocabulary_service
vocabulary_service_term
(NOT to be confused with the CKAN core vocabulary table)
A cron job needs to be set up to refresh the vocabulary sevices periodically based on each vocabulary_service.update_frequency setting.
ckan -c path/to/ckan.ini vocabulary-services-refresh