You most likely need to enabled port 80 forwarding from your gateway.
This is usually done by logging into the gateway device via its web interface, and adding a forwarding in the "Port forwarding" or "NAT" menu. You will be able to find specific instructions for your device on the manufacturer's website.
Another issue is if you don't have a static public IP. If you're using a home, or low-end business, internet connection, which I assume since you're using Vista business as a server, you probably have a dynamic public IP. If you do you'll need to use a service such as DynDNS.com to create a url which gets updated whenever your public IP changes.
Yet another issue is that using Vista Business as a comercial web server violates the EULA. If you intend to use this as a production web server you should look into Windows Server 2008 Web Edition.