A Virtual Private Network creates an "intranet" by tunneling through the Internet. You must create a VPN Server and each client's Web Browser must be configured to recognize the VPN. If all your users are known, and they use the same PC to connect, a VPN can be the most secure method. If your corporation already uses a VPN, then you can utilize its VPN server and existing clients (assuming they are Internet Explorer 6.0, 7.0, or later). The client web browsers must be configured to connect with your VPN server.
VPN do not provide the speed of a non-tunneling protocol, and users will see a slower response. VPN have large overhead and result in very slow response for clients connecting remotely via the VPN. Placing your WebAccess SCADA node behind a firewall/router with Private IPs, then using NAT with port mapping and using HOST files for Public and Private users is preferable to using a VPN.