Access localhost on iPad
You don't want to access localhost on your iPad because you're not running a webserver on your iPad. I'm not sure if I understand the question, but it sounds like you are running the webserver on you Windows machine and you can access it on that machine at http://windows-pc-name.

I'm not entirely sure what you mean by "the app server requires a VPN connection," but it sounds like you need to connect to the remote network in order to run the webapp. Generally all traffic is sent over VPN so that is probably why you can't visit http://windows-pc-name on your iPad's browser. I would see if you can configure your webserver to use VPN, and not have your Windows network connection go through VPN. That way the server can communicate with the remote network, but it won't interfere with anything else.

Just in case you are using the wrong name, try http://192.168.1.101 or whatever you Windows Machine's local IP address is.