If I AirPlay something from my laptop, which connects to the internet via VPN, to my Apple TV which connects to my wifi router which is NOT VPN protected, is the content I'm AirPlaying exposed beyond my local network?
No, the airplayed content won't leave your local network.
Your VPN only protects traffic that goes through your internet gateway which is usually your modem/router. (In the less common case that it is a split tunneling VPN, even only the traffic to the remote LAN is affected by the VPN.) However, Airplay connects your laptop with your AppleTV within the local network, so it is neither affected by the VPN, nor does it leave your local network.