if qubes settings tells you:
ip: 10.137.0.123
netmask: 255.255.255.255
gateway: 10.137.0.1
dns: 10.139.1.1, 10.139.1.2
you need to have:
in /etc/hostname.xnf0:
inet 10.137.0.123 255.255.255.0
in /etc/mygate:
10.137.0.1
in /etc/resolv.conf:
nameserver 10.139.1.1
that should be it
then run sh /etc/netstart xnf0
forget about vlan0
sometimes the nic gets stuck. just reboot the qube if config is correct but cant ping default gw or 8.8.8.8