http://kagarlickij.com/windows-10-connected-vpn-dns-issue-solution/

set-VpnAdapterMetric.ps1

<#	
	.NOTES
	===========================================================================
	 Created on:   	Jan 2017
	 Created by:   	Dmitriy Kagarlickij
	 Contact: 	    dmitriy@kagarlickij.com
	===========================================================================
	.DESCRIPTION
		This script should be executing with Administrator-level permissions
#>

$mainIfName = "Ethernet"
$vpnIfDescription = "TAP-Windows Adapter*"

$mainIfIndex = $((Get-NetAdapter | Where-Object {$_.Name -eq $mainIfName}).ifIndex)
$vpnIfIndex = $((Get-NetAdapter | Where-Object {$_.InterfaceDescription -like $vpnIfDescription}).ifIndex)

$mainIfMetric = $((Get-NetIPInterface | Where-Object {$_.ifIndex -eq $mainIfIndex -and $_.AddressFamily -eq 'IPv4'}).InterfaceMetric)
$vpnIfMetric = $((Get-NetIPInterface | Where-Object {$_.ifIndex -eq $vpnIfIndex -and $_.AddressFamily -eq 'IPv4'}).InterfaceMetric) 

if ($vpnIfMetric -ge $mainIfMetric) {
    Write-Output "VPN DNS will be fixed now"
    Set-NetIPInterface -InterfaceIndex $vpnIfIndex -InterfaceMetric $($mainIfMetric-5)

    $mainIfMetric = $((Get-NetIPInterface | Where-Object {$_.ifIndex -eq $mainIfIndex -and $_.AddressFamily -eq 'IPv4'}).InterfaceMetric) 
    $vpnIfMetric = $((Get-NetIPInterface | Where-Object {$_.ifIndex -eq $vpnIfIndex -and $_.AddressFamily -eq 'IPv4'}).InterfaceMetric) 

    if ($vpnIfMetric -lt $mainIfMetric) {
        Write-Output "VPN DNS has been fixed successfully"
    } else {
        Write-Output "VPN DNS has been fixed with error"
    }

} else {
    Write-Output "VPN DNS is ok"
}