As suggested, I configure the default  host with server ip, and a blank start page.
Hostname = xx.xx.xx.xx
WebsiteRoot = /var/www/hiawatha
StartFile = index.html
AccessLogfile = /var/log/hiawatha/access.log
ErrorLogfile = /var/log/hiawatha/error.log
Then, a virtual host.
VirtualHost {
    Hostname = censored.com, www.censored.com
    WebsiteRoot = /data/www/censored.com
    AllowDotFiles = no
    AccessLogFile = /var/log/hiawatha/censored.com/access.log
    ErrorLogFile = /var/log/hiawatha/censored.com/error.log
    UserWebsites = no
    #UseFastCGI =
    #UseToolKit =
}
When requesting "http://censored.com", works as expected.
When requesting "http://www.censored.com", I get a blank page. Not expected.
When requesting "http://censored.com/somedir", works as expected.
When requesting "http://www.censored.com/somedir", works as expected.
Hiawatha version: 9.6
Operating System: Debian Wheezy (amd64)