first off TL;DR:
Windows assigns metrics automatically, when a connection is faster than the virtual VPN Adapter's stated speed, you don't get DNS resolution for internal networks.
We had DNS issues for several customers that boiled down to the virtual NAC VPN adapter metric.
Basically, this metric is used for DNS requests in Windows 7/10 and we had to change it to a value that would make sure the NAC VPN adapter is always preferred when a VPN connection is established. Otherwise, connections faster than 100mbit/s (NAC VPN adapter speed) would get used to resolve DNS and thus our users local Modem/Router would try to resolve company internal hostnames using external DNS servers. I specifically refer to the adapter metrics for IPv4 and IPv6 Interfaces on the NAC VPN Adapter, not to any routes.
Number of cases seems to increase because of home office users' wifi connections, as well as notebooks' wifi adapters, getting faster. We had one out of >1000 users connecting over LAN from home, same issue.
So my feature request: Add a switch for NAC client installation to set the metric value for the NAC VPN adapter. Maybe even set a default value of 1 or 2 for the metric.