How to force Vagrant to have a single, bridged, network interface

You can try to add the adapter: 1 to the config. Although I still find some problem with it. See if it will help you starts from somewhere.

Example:

config.vm.network "public_network", bridge: "Broadcom BCM5709C", adapter: "1", ip: "192.168.x.xx"

You will still have the eth1 which is host-only network...