CHEYENNE, Wyo. - About 100 Taco John's franchises in the Midwest are switching to a different fresh produce vendor as a precautionary response to reports of E. coli illnesses at three of the franchises, officials said yesterday. The three restaurants, one in Iowa and two in Minnesota, used a common vendor. Company officials yesterday said they were aware of more than 50 people in Iowa and Minnesota who reportedy got sick after eating at one of the three restaurants, among other places. Investigators had not determined the source of the illnesses, but officials at the taco chain said they decided to replace the produce vendor as a precaution. Based here, Taco John's International operates and franchises more than 400 quick-service restaurants. At the same time, public health authorities continued to investigate a separate outbreak of E. coli infections involving Taco Bell restaurants on the East Coast.