I'm not sure that's the way to do it. I mean, I do use forwarded ports on my projects and I also configure my private network for each one but at least on my current machine it doesn't allow outside access unless I mess with the router and the firewall. A little bit easier would be to configure a network as "public_network" on the Vagrantfile. I assume this works for most people but, again, my machine doesn't vagrant up if I have a public_network configured. I never tried to figure out why, it's not something I had to care about that much to be honest.

So, as you can see, if vagrant share works as announced it would make it much easier for me for that particular task.