How to use a firewall AND a VPN? I'm using the only open source firewall for Android: NetGuard and would like to use a VPN in addition. Is this possible? If so: how?

Both types of apps typically use virtual VPN tunnels. NetGuard has network settings too. Many adblockers use such as well which is why I'm only using a Firefox AddOn.
Quoting the NetGuard FAQ on that topic:


  Can I use another VPN application while using NetGuard?
  
  If the VPN application is using the VPN service, then no, because NetGuard needs to use this service. Android allows only one application at a time to use this service.
  
  NetGuard is a firewall application, so there is no intention to add VPN support. However, NetGuard supports a SOCKS5 proxy to chain VPN applications.


So the answer is: it depends.