this is a sample vhost:
====
<VirtualHost *:80>
  ServerName example.dev
  ServerAlias *.example.dev
  DocumentRoot "PATH/TO/ROOT/OF/EXAMPLE"
  <Directory "PATH/TO/ROOT/OF/EXAMPLE">
    AllowOverride All
    Allow from All
  </Directory>
</VirtualHost>
====

notice the *.example.dev, that will allow apache to route every subdomain of example.dev to the root of example. Now, if you are on windows, sadly the hosts (C:\WINDOWS\system32\drivers\etc\hosts) file doesn't allow you to do wildcard subdomains, so you have to add all the subdomains you want to use:

127.0.0.1   example.dev            # example.dev development
127.0.0.1   subdomain.example.dev  # example.dev subdomain

you can validate the vhosts by doing:

$ httpd -S