It really depends what you want do do.
I here run Zeroshell on Virtualbox too, and it is bridged one of my network adapters. I use it to make VPN connections to physical Zeroshell routers. Works fine.
I don’t know anything about Radius, I don’t use it.
Maybe you have some kind of limit in the network where your Zeroshell is bridged? Maybe MAC address count limit or lock or comething?