By default, Vilfo use the DNS addresses provided by your ISP when not connected to a VPN, but you can manually change the DNS used by Vilfo over WAN.
When you connect a group to a VPN tunnel, your devices in that group will no longer use the DNS addresses provided by your ISP. Instead, they will use the DNS servers provided by your VPN. This means that if you have several groups with different VPN connections, they will all use different DNS addresses.
Do note that there is currently a limitation in Vilfo where if several VPN servers use identical DNS addresses, only the DNS server provided by the last VPN server will be used.