Using variables in Apache config files to reduce duplication?

You could use mod_macro, which has been included in Apache httpd since version 2.4

Before that it had to be installed separately, see mod_macro. For example on Debian: apt-get install libapache2-mod-macro; a2enmod macro.

Example configuration

/etc/apache2/conf.d/vhost.macro

<Macro VHost $host $port>
  <VirtualHost $host:$port>

    ServerName $host
    DocumentRoot /var/vhosts/$host

    <Directory /var/vhosts/$host>
      # do something here...
    </Directory>
  </VirtualHost>
</Macro>

/etc/apache2/sites-available/vhost.mysite.com

Use VHost vhost.mysite.com 80