Enable SSL

[kidkl]

It's my understanding that you can install memcached and configure vbulletin to store session information in that. I am also assuming that you are running a master-slave mysql configuration and not master-master. So you can also setup replication for memcache to transfer the cache over to your backup memcache instance on your mysql server. I personally have very little experience with memcache but I believe you can also create a pool configuration that is active-active.

As far as the load-balancer is concerned, I assume that you would be using SNAT on the F5 to load-balance between the servers at the different locations? If that's the case you can use a pound/haproxy configuration to terminate the ssl connection and then talk to the real servers via http. This configuration could be done on the same server as the webserver if need be.

If you don't want to install extra software, you can also use apaches mod_proxy balancer to load-balancing, or hot standbys. It will do the x-forwarded-for header as well as cookie injection for persistence.