A critical moment in the government’s case against Robert Kaluza, who was facing a criminal charge for his role in the 2010 BP PLC oil spill in the Gulf of Mexico, came when a former colleague was called to testify. Donald Vidrine had already pleaded guilty and was expected to bolster prosecutors’ arguments. But he had trouble articulating exactly what he—and by extension Mr. Kaluza—may have done wrong. “I, we, uh, I may…