If you take apart the Vex ultrasonic, it is practically exactly the same as the Devantech SRF04 (
http://www.acroname.com/robotics/parts/R93-SRF04.html). If using it with easyC, I'm nearly 100% certain that the acroname one will work fine.
As far as the encoders, I would use a quadrature encoder instead of the vex encoders, but I guess you can't beat their price.