I use several supplements and many companies used terms like: "manufactured for" and "manufactured by" without saying precisely where the product was manufactured.
China has become very big in producing supplements and prescription drugs:
Chinese pharmaceutical companies also have taken over much of the world market in the production of antibiotics, analgesics, enzymes and primary amino acids. According to an industry group, China makes 70 percent of the world's penicillin, 50 percent of its aspirin and 35 percent of its acetaminophen (often sold under the brand name Tylenol), as well as the bulk of vitamins A, B12, C and E.
http://www.google.com/search?hl=en&q=China+Vitamin+C+market&sourceid=navclient-ff&rlz=1B3GGGL_enUS345US345&ie=UTF-8
I think some US companies buy the bulk materials and still encapsulate in the US.