* Greece elections add to euro zone's uncertainties * Cognizant slumps after results * Vertex jumps on cystic fibrosis drug study results * Dow off 0.2 pct, S&P flat, Nasdaq off 0.1 pct By Chuck Mikolajczak NEW YORK, May 7 (Reuters) - U.S. stocks declined on Monday as investors had a muted reaction to European election results that cast doubt on the region's ability to tackle its debt crisis. The election results initially roiled futures markets on Sunday night, but