It depends on your server software. Since you mentioned httpd.conf, I'm taking it to mean that you run Apache on Linux distribution. If that's true then adding virtual host sure is enough.
One way of doing it is:
1. Get a domain, purchase if you want or get a free domain online ( freenom.com can be used to get a free domain)
2. Identify the external IP or DNS for your EC2 instance. Associate an Elastic IP with your instance or the IP of your instance will change on reboots.
3. Create a DNS record for your domain
4. Ensure that your httpd.conf contains a line to allow virtual hosts:
5. Create a virtual host directive as shown below:
ServerAdmin [email protected]
Options Indexes FollowSymLinks MultiViews
allow from all
CustomLog /var/log/apache2/subdomain.example.com.access.log combined
6. Restart Apache and you are good to go.