I imagine you can use mencached. I've used it heavily on very large Magento ecommerce stores in tandem with APC. Haven't looked at integrating it with vbulletin.

We won't have a pair of F5s any time soon, so discussing layout there is kind of pointless. One of the servers is leased, so we can't just throw hardware into the rack like we can the second.

MySQL is setup master-master. Has to be for load balancing. Master-slave would only work for failover, and then carries a slower recovery time back to primary (no hot sync). PHP sessions are stored in the DB and cookie, so not much problem there. The only
Potential concern is the SSL session. F5s have the ability to replicate SSL sessions as well, but currently we can't. With newer browsers, it shouldn't be a large concern. They should renegotiate the SSL session on their own, though there will be some slight delay (timeout wait) after a failover occurs for users. Old browsers will error, an the refresh button will have to be pushed.