By Michelle Martin BERLIN (Reuters) - German's Social Democrat vice chancellor, Sigmar Gabriel, warned on Friday that a dispute among the conservatives over how to handle record inflows of migrants into the country was jeopardising the government's ability to act. Merkel and Horst Seehofer, the leader of her Bavarian sister party, have frequently clashed over how Germany should deal with the estimated 800,000 to 1 million it expects this year, many of them from war zones in the Middle East. Merkel, the leader of the Christian Democrats (CDU), has favoured an open-door policy and says Germany can cope.