LONDON, May 12 (Reuters) - U.S. sports tycoon Malcolm Glazer agreed to buy a majority stake in Manchester United and launched a 790 million pound ($1.5 billion) offer for the rest of the English soccer club, his bid vehicle said on Thursday. Red Football Ltd, acting on behalf of Glazer, said it had agreed to buy a 28.7 percent stake in Manchester United from Irish race horse magnates John Magnier and J.P. McManus for 300 pence a share to add to his existing 28.1 percent stake. arty: