By Nandita Bose, Yomna Ehab and Jonathan Allen 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.