You can move time entries from one vendor to another in the same way as you do it for employees. The simplest way is to use the Batch Update feature and update the Employee ID of the entries. Check out this article for all the details: Moving time and expenses to another employee.
Note: You need to make sure that the option Include vendors in employee drop-downs is checked in User Preferences > General. Otherwise, the Employee drop-down in the Batch Update dialog only shows vendors of type Outside Consultant and Contract Employee.