Just use your dyndns hostname for the virtual host and your local IP (or just 127.0.0.1, the hostname of the default host is not really important) for the default host.
In detail: When you request a website with your browser, the domain part of the url you entered (hostname) is compared with the hostname settings of all virtual hosts and the matching one is selected. If there is no match, the default host is used. So if you want to website to by available over http://myhost.dyn.any/* then your hostname should be "myhost.dyn.any".