BILLINGS The Environmental Protection Agency has released a finalized plan to prioritize testing and mitigation of a chemical vapor plume in parts of Billings, Montana a cleanup effort that has been ongoing for more than 30 years.The plan will focus on approximately 750 buildings in a designated area of the city. The contamination stems from a dry cleaner on Central Avenue near 7th Street West, where perchloroethylene commonly known as PCE seepe…