If you want to get your hands dirty, so to speak, a great place to start would be a PIC 16F877 kit offered by Machine Science, a new FIRST supplier. It will give you six 10-bit analog channels and a bunch of digital IO. The chip itself is only $10, so you could have multiple co-processors for different modules. It would be feasible, for example, to have a dedicated processor for handling high-resolution encoder interrupts and then reporting to the FRC. Their online IDE and programmer (free for FIRST teams) make it very easy to work with and it can communicate with the FRC via USART (the TTL or programming port).
I am sure you can use the ttl but most teams prefer to use that for the camera. You can use the programming port.
However If you are going to go through the trouble of getting a co-processor working atleast make it worth while, a vex controller really doesn’t give you much of an advantage, I would look into better systems (gumstix for example.)