Your comments

The vhost file doesn't actually get created when you create a new page, though, but only when you actually assign a domain to a page. Ramiro's point about how this would automatically add a www.subdomain.example.com to each subdomain, is a good one. I'll just keep adding them manually for the time being. Thanks.
[root@web01 plugins]# grep server_name /var/lib/ajenti/plugins/vh-nginx/nginx.py
'server_name': (
'server_name %s;' % (' '.join(domain.domain for domain in website.domains))

Does anyone know how to adjust this so that it automatically adds the www subdomain to each vhost?  Right now I need to add the www as a separate domain in Ajenti for each domain added.

Should look like

server_name www.example.com example.com;