Radius configuration

First, I assume the serves work by themselves, right? If so, you could isolate the pool from the virtual by configuring the specific port and adding a radius monitor, with a username and password. This will test connectivity from the self IP to the servers. If the servers are UP and still failing through the VIP, then you have likely isolated the problem through the virtual. Test basic connectivity without the persistence to eliminate that. Use tcpdump constantly to understand what traffic is flowing where until the problem is found.