WASHINGTON/DUBAI, June 19 (Reuters) - Switzerland said U.S. talks with Iranian negotiators on a pact to end the Middle East conflict would not take place on Friday, as Vice President JD Vance dropped plans to travel to Geneva, adding to uncertainty whether a lasting truce can be found. "The logistics of these negotiations have never been simple or predictable," the White House spokesperson said in a statement on Thursday night. Vance and the U.S. delegation had been ready to depart as soon as plans were finalised. The talks, set for the mountaintop resort of Burgenstock, would not take place, Switzerland's...