Where macOS keeps VPN Network configuration?
When using the Default profile with Configuration: for the VPN and modifying its settings, the /Library/Preferences/com.apple.networkextension.plist file, the one starting in the root of the e.g. Macintosh HD, not your Home folder, will contain the VPN settings.

Also note that if you select Add Configuration... with Configuration: for the VPN, then some information is also stored in the ~/Library/Preferences/ByHost/com.apple.networkConnect.&lt;UUID&gt;.plist file for the VPN as well.

By the way, you can use the defaults command e.g. defaults read /Library/Preferences/com.apple.networkextension.plist, to read this and other .plist files, you do not necessarily need to use plutil to convert to XML in order to read them.