Sorry i don't really know how to phrase the question, perhaps let me explain the scenario.
Say I own the domain xyz.com, and I'm using AWS Route 53 as my DNS.
I want to have someInternalStuff.xyz.com resolvable to an internal IP address when I connect to my VPN
I have setup a VPN server and a DNS server, and when I connect to the VPN, it pushes the DNS server to me. That's great, I setup bind9 to act as the authoritative server for the zone xyz.com., when I connect to the VPN, I can dig someInternalStuff.xyz.com. But is there a way I can configure my dns to fall back to DNS servers out on the internet if it can't answer the query, say www.xyz.com even though it's the authoritative dns for xyz.com?