Can the Vice President of the United States order the Secret Service to hold witnesses in protective custody?

No, the Vice President of the United States does not have the authority to order the Secret Service to hold witnesses in protective custody. The Secret Service is primarily responsible for the protection of the President, Vice President, and certain other individuals, and its operations are guided by federal law and directives from the President and other senior officials. Protective custody decisions typically involve law enforcement agencies and judicial authorities, rather than being directed by the Vice President.