The following configuration can enable the rewrite rules of your virtual hosts and make robots.txt point to any other dynamic file (e.g. robots.php or robots.asp) which can change content dynamically.
Add rewrite rules into <Directory> within the Virtual host.
[root@test ~]# vi /etc/httpd/conf/httpd.conf
...
<VirtualHost *:80>
DocumentRoot "/var/www/html/example"
ServerName www.example.com
...
<Directory "/var/www/html/example">
AllowOverride All
RewriteEngine On
RewriteRule ^robots.txt$ robots.php [NC,L]
</Directory>
...
</VirtualHost>
Validate the syntax of httpd.conf.
[root@primary01 ~]# apachectl -t
Syntax OK
Restart httpd service.
[root@primary01 ~]# service httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]
Now you can verify your robots.txt. For example:
http://www.example.com/robot.txt