By Kate Holton and Paul Sandle LONDON (Reuters) - Vodafone will reinvest a $3.2 billion (2.1 billion pounds) dividend from its healthy U.S. arm to counter weakness in southern Europe that contributed to the largest ever quarterly fall in the group's main revenue measure. The British firm is trying to decide whether to sell Verizon Wireless, its profitable U.S. unit in what could be the world's third largest deal to support its struggling core operations. ...