Is it possible to access the PIC18F8520 I2C and SPI peripherals? I would like to be able to connect I2C based temperature and humidity sensors and SPI based Vector IIX Electronic Compass to my Vex applications. I also want to network multiple Vex controllers for parallel processing. Ideally these I/O pins would be brought out to the back of the Vex Controller.

No. All interfacing must me made through the Analog/Digital, Interrupt, and Serial Ports. You can talk to other Vex Controllers via the 2 Serial Ports.**

Do you have any advice about how to access IC2 based sensors with the Vex?

The I2C Bus is not available to the User. The Advanced Programmer can “Bit Bang” the interface using some of the Digital Control lines, but we do not support this effort.