public class CourseEvent : ICourseEvent
{
    // other properties
    public string Vendor { get; set; }
}

public class CourseVendor
{
    public string Name { get; set; }
}

// 1) get List<CourseEvent> items
List<CourseEvent> courseEvents = LoadCourseEvents();

// 2) group items by property "Vendor"
IEnumerable<IGrouping<string,CourseEvent>> groups = courseEvents.GroupBy(c => c.Vendor).ToList();

// 3) convert to list
List<CourseEvent> courseVendors = groups.SelectMany(group => group).ToList();

// 4) initiate target class
List<CourseVendor> vendors = new List<CourseVendor>();

// 5) fill target class
courseVendors.ForEach(c => vendors.Add(new CourseVendor { Name = c.Vendor }));