1) You must use vnIPCfgIface to configure your IP addresses before you send or recv across vrnet. This means that you will need to specify IP addresses for each interface using some method of your choosing.
2) You can download an example here.
3) Make sure that before you recieve your packet with vnIPRecvPkt, you have allocated enough memory to put the packet into. To be safe, you can allocate BUFFSIZE, as defined in vrnet.h.