Upon reflection, I suspect those localhost-vpn ad blockers have to reconstruct your ip traffic back into outgoing socket api calls, which can't work really well for non-web traffic.
I .. only learned about these ad blockers from this thread, so take everything I write with a healthy amount of table salt.
That said, as far as I understand this DNS66 thing only intercepts DNS requests (which yes, it needs to break apart and reassemble) and nothing else? That is - if you don't talk to a server that is part of the application's custom DNS server list or a DNS server given to you by your network configuration, then I'd expect this packet to bypass the VPN entirely.
Maybe someone who understands the Android VPN service thing a bit more can chime in and confirm?
