To get a sense of how vulnerable the U.S. economy could be if the euro currency union cracks apart, start with the volume of U.S. exports to the euro zone — $153 billion in the first six months of the year. Add several hundred billion dollars in investments by U.S. banks in the euro zone and several trillion dollars’ worth of other financial contracts between the two economies. As European leaders meet later this week to try to resolve their spreading debt crisis and prevent the breakup of the 17-nation euro zone, U.S. politicians, corporate leaders and financial analysts are watching anxiously for a breakthrough.