Modern Robotics now has extension cables and pigtails:
http://www.modernroboticsinc.com/sensor-pigtail-kit
http://www.modernroboticsinc.com/sensor-extension-kit
Note that the Vex extension cables may need to have a side tab cut/sanded in order to fit in the connector. I haven't ordered any of the Modern Robotics extensions, but I'm guessing they don't have this issue.