BEIJING, Oct 10 (Reuters) - Vehicle sales in China fell 1.8 percent in September from a year earlier as Japanese automakers' sales collapsed, adding to downward pressures from a slowing economy and rising fuel costs that have weighed on the world's biggest auto market. September's was the first monthly fall since January 2012, raising the risk of a deeper slump which would harm Chinese and foreign carmakers alike. A territorial row between China and Japan, which spurred violent protests in some parts of China last month as well as calls for boycotts of Japanese products, hammered Japanese car sales, a shift from which European and South Korean firms have been the chief beneficiaries.