The troubled European car market is now dragging down even Volkswagen, the region's leading automaker, as vehicle sales on the Continent have declined to their lowest levels in decades. Until now, Volkswagen, the German auto company, had been buffered a bit more than other auto companies doing business in Europe because of its size and strong sales in North America and China. But its shrinking profit margins reflect both the industry's steep sales decline in Europe as well as intense price competition in the biggest vehicle segments. Volkswagen joins a growing roster of foreign and United States automakers that are struggling in Europe, where car sales dropped 10 percent during the first quarter, including double-digit decreases in France, Germany and Spain.