Load balance is generally about spreading the "cpu load" among several servers to best utilize those cpu resources. However, for some people load balance could be considered a way to best utilize the available bandwidth on multiple ISP connections. Yet another method of load balance could be to balance outbound calling through several carriers.
You would appear to be asking about the first but somehow linking "load balancing" with "can I share an IP address between servers". Which ... means you are likely overcomplicating a scenario by asking about one scenario at once (which is often necessary when planning a change such as this ...). Handle it in stages with clear breaks between them when asking the questions and it may get easier.
Can you have one IP address with multiple phone servers (dialers)? Yes. Of course. The dialers would have LOCAL IP addresses instead of using the public IP. A router would have the public IP. The dialers would use that router as their Gateway.
If you have multiple servers behind a single IP, can they receive and make calls and have agents register their phones to them? Yes. Agents on the same local subnet would access the dialers through their local IP. Agents who have to access through the public IP (now through the router), however, would have a bit more of a challenge. One server could easily be set to accept this traffic on port 5060, but for a second server this would need a second SIP port (such as port 5061) or a very complex router configuration (which I've never personally attempted, to be clear).
For inbound phone calls, this same restriction applies EXCEPT: if your server will be registering to the inbound carrier, this would allow multiple servers on that single router to register through the router to receive calls. This is because the act of registering allow a completely different port to be used for each registration, removing the "port 5060" problem. No help for agents, but great for carriers sending calls to different servers on the same public IP.
That being said, if you put an asterisk server at a remote location (such as a call center in another state/country) and registered your vicidial server to that server, your agents could access Vicidial through sip or iax accounts on that server.
Which raises another point, if you have 50% of your agents on SIP pointed to one server via port 5060, you could have the other half of your agents pointed to the other server via IAX2 on port 4569. This all assumes that your router has a sip algorithm to make SIP viable through two routers -- one at the vicidial server and one at the agent location (as technically sip is incapable of this feat so most routers now have technology in them to assist).
Clearer?
