How to use a firewall AND a VPN?

I'm using the only open source firewall for Android: [NetGuard](https://play.google.com/store/apps/details?id=eu.faircode.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.