President Clinton surely will take credit for this statistic: Sales of Big Three vehicles in Japan, including those produced in Europe and Asia, are running 40% ahead of last year's pace. Through April,, GM and sales reach 40,216, up from 28,798 through the first four months of 1995. Sales of U.S.-built vehicles are up 52% to 24,776 for the same period. That still leaves a long way to the 300,000 vehicles a year the Big Three expect to sell in Japan by 2000. For the first quarter, all imports account for only 5.7% of Japan's market, roughly the same as last year. The Big Three's collective share is only 1.6%.