How to set the default virtual host on Apache 2?

I have the WordPress one click install app image and I need to fix some problems with the default server.

I have two sites enabled. Let’s call them site-a and site-b with the requisite config files at /etc/apache2/sites-available/site-a.conf and site-b.conf. Both sites are enabled so symlinks to these files appear at /etc/apache2/sites-enabled/

Both sites are running as they should except for one big problem - apache is closing one of the two sites to be the “default” site at random and it’s choosing exactly the one I don’t want.

Nothing I’ve tried is fixing this. I thought that I just needed to put in the top line of the site I want to be the default this: " <VirtualHost default:80>" but it isn’t working.

What do I have to do here?

Here are my two config files, nothing special.

Site A:

<VirtualHost *:80>
	ServerName site-a.com
        ServerAlias site-a.com www.site-a.com
        ServerAdmin webmaster@site-a.com
        DocumentRoot /var/www/html/site-a.com

        <Directory /var/www/html/site-a.com>
            Options Indexes FollowSymLinks
            AllowOverride All
            Require all granted
        </Directory>
        
        ErrorLog ${APACHE_LOG_DIR}/site-a_error.log
        CustomLog ${APACHE_LOG_DIR}/site-a_access.log combined
</VirtualHost>

Site B:

<VirtualHost *:80>
	ServerName secondary.site-a.com
        ServerAdmin webmaster@secondary.site-a.com
        DocumentRoot /var/www/html/site-b.com

        <Directory /var/www/html/site-b.com>
            Options Indexes FollowSymLinks
            AllowOverride All
            Require all granted
        </Directory>
        
        ErrorLog ${APACHE_LOG_DIR}/site-b_error.log
        CustomLog ${APACHE_LOG_DIR}/site-b_access.log combined
</VirtualHost>

Now all I want is for the server to serve up Site A when in doubt. I only want it to serve up Site B when somebody enters exactly http://secondary.site-a.com or http://secondary.site-a.com/some/url/at/subodain/secondary

The problem is it’s doing the opposite. How do I fix this?

Thanks.