I called my senator's office after reading this and his office said that the CODEX/WTO agreement is "not binding in any way." I asked the staff-person what Mr. Durbin's views of regulating supplements like vitamin C and she told me that he is against it.
He did however support (or draft?) a bill that requires supplement retailers to notify the proper authorities if someone reports a health problem after using a supplement purchased from that retailer.
Basically her opinion was that supplement regulation won't get very far in the USA. She did mention "dangerous stimulants like ephedra" were a different story though.