WPILib is the framework written by Brad Miller at WPI (hence the name) from which EasyC is based on. You can program using WPIlib in either EasyC PRO (because it supports custom .c files and has a built-in text editor) or any other IDE of your choice. (MPLab, Eclipse, etc). Every time you use something like
StartUltrasonic(); or
SetPWM(); in EasyC, that's really a WPILib function.
There are currently 2 serial ports on the Vex controller - the RxTx port and the programming port. Using WPILib, you can interface with TTL devices (like the CMUcam) using the RxTx port and a
TTL converter. As long as the device is 5v and outputs TTL, you should be able to write your own custom 'drivers' to read the signal on the Vex controller using WPILib.