Unfortunately the puppetlabs/apache module does not support this directive without enabling SSL on the vhost, so I had to manually generate the config file.
After an hour, I had the configuration in place but starting Apache gave me the following output:
$ /etc/init.d/apache2 restart [....] Restarting web server: apache2no listening sockets available, shutting down Unable to open logs Action 'start' failed. The Apache error log may have more information. failed!
The log directory /var/log/apache2/ was really not owned by www-data, so I fixed that. I tried to fix many more things, and finally strace'ed the apache process: Still the same error message, but not a single access to a log directory or file.
The real cause of the problem was that my apache configuration did not include a Listen directive, which meant that my vhost, although defined, was not used.
Still: a message Unable to open logs is hardly proper for a missing Listen line in the config.