Well, figured out why my friends couldn't log into my my computer via my new dynamic dns address -- it was my digital phone modem. Apparently, this acts as a router blocking my machine's ip address from the public.
I suppose I'll get in touch with the Vonage people and ask them how to solve this without disconnecting it.
Right now, however, I can get to my machine with my new virtual host fine:
Check these people out. It's free and you download their free client that tracks the dynamic IP address generated by your ISP then match it to to virtual domain name you choose.