How exactly does a VPN work for iPad?
The VPN is a function of the specific version of iOS and not very well documented in the user guide. The information needed to establish a connection is more a function of the configuration on the VPN server and less about how the iPad data gets entered (except the details must match for the VPN to work)

A better resource is the iPhone configuration utility (more of a total iOS configuration tool) and enterprise deployment guide that goes into much more detail on what VPN protocols and settings can be used with iPad. They have step by step guides on setting up a VPN or having iOS devices join existing VPN. 

http://www.apple.com/support/iphone/enterprise/