It looks like you have two problems here:
(1) "vlan 0 with no nics" -- this means you've created a network (with -net) but haven't plugged any emulated network cards into it. QEMU's networking configuration is in two parts: the backend where you say how the network should work, and the frontend where you create network cards and plug them into that network. The default command line arguments will do both for you, but if you specify any networking options the defaults don't kick in, and so you need to specify both parts yourself. That means you need either
(a) new style command lines with -netdev for the network and -device for the nic
(b) old style legacy command lines with -net user/tap/etc for the
network and -net nic for the nic
Check the documentation for details: http://wiki.qemu.org/Documentation/Networking
(2) Your other problem is that it looks like you don't have a working X display, which means that QEMU is failing to create the graphics window. This is the actually fatal error -- messing up your network config will just create a guest which isn't connected to the net.
Also, your error message seems a bit scrambled: the "Warning: " part of the message goes in front of the "vlan 0 with no nics" part. Cut and paste mixup?
