Vinyl chloride, the cancer-causing chemical which menaces the plastics industry, again has roused industrialists and environmentalists to fierce debate over when enough is enough.

In open hearings yesterday, industry spokesmen angrily denounced as unnecessary, useless and too costly strict new standards to limit the emission of vinyl chloride.

But an attorney for the Environmental Defense Fund, the leading proponent for federal action, claimed the new regulations - the second set of emission standards to be strapped on the industry in nine months - are necessary to jolt plastic manufacturers into developing more efficient cleaning technology.

Until 1974, vinyl chloride was accepted as a cheap and almost innocuous gas used as a propellant for insecticides and hair sprays and as a base ingredient in a common plastic (polyvinyl chloride) found in flooring, car upholstery, wrapping, pipes, phonograph records, film and other products.

In recent months, however, the chemical has been blamed for the cancer death of at least a dozen industrial workers. Several studies suggest prolonged exposure to vinyl chloride can cause a rare and fatal form of liver cancer in the hundreds of employees who work in, and 4.6 million people who live within five miles of the nation's 58 manufacturing plants.

Standards proposed by the Environmental Protection Agency would reduce the acceptable amount of chloride from 10 to 5 parts per million in air or water discharges.

Though the reduction appears significant, Ralph Harding, president of the Society of the Plastic Industry, complained it would do little to improve air quality because of the relatively larger amount of gas that would continue to escape through uncontrolled points in the manufacturing process.