QinQ — allows multiple VLAN tags to be inserted into a single frame.

QinQ can be used to tunnel vlans in a vlan.

vif-s and vif-c stand for the ethertype tags that get set:
The inner tag is the tag which is closest to the payload portion of the frame; it is officially called C-TAG (Customer tag, with ethertype 0x8100).
The outer tag is the one closer/closest to the Ethernet header; its name is S-TAG (Service tag, ethertype 0x88a8). Ethertype for service tag might be redefined by cli command.

Example:

vyos@vyos-rtr:~$ configure
vyos@vyos-rtr#set interfaces ethernet eth0 vif-s 333
vyos@vyos-rtr#set interfaces ethernet eth0 vif-s 333 address 192.0.2.10/32
vyos@vyos-rtr#set interfaces ethernet eth0 vif-s 333 vif-c 777
vyos@vyos-rtr#set interfaces ethernet eth0 vif-s 333 vif-c 777 address 10.10.10.10/24

Configuration commands:

interfaces
    ethernet <eth[0-999]>
        address <ipv4>
        address <ipv6>
        description <txt>
        disable
        ip
            <usual IP options>
        ipv6
            <usual IPv6 options>
        vif-s <[0-4096]>
            address <ipv4>
            address <ipv6>
            description <txt>
            disable
            ip
                <usual IP options>
            ipv6
                <usual IPv6 options>
            vif-c <[0-4096]>
                address <ipv4>
                address <ipv6>
                description <txt>
                disable
                ip
                    <usual IP options>
                ipv6
                    <usual IPv6 options>