The vex 29 motor controllers should work if you plug them into the roboRIO just like a talon or what ever, they all send the same signal, if it doesn't work, its probably because the power bus the RIO is sending it is to low of current/voltage. All of the PWM motor controllers use the same signal.
I found this image on google, this is how you should wire it.
http://4.bp.blogspot.com/-Xzhvsr9ip2...0/modified.png
make the new Vin and Gnd wire going to the Power distro board