ViewModel => TaskScheduler => SynchronizationContext => WPF (Win RT) Dispatcher

var dispatcher = Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher;
await dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () => { });

    public static class TaskHelper
    {
        // Вызываем при старте приложения
        public static void InitializeSyncScheduler()
        {
            SyncScheduler = TaskScheduler.FromCurrentSynchronizationContext();
            SyncFactory = new TaskFactory(SyncScheduler);
        }
 
        public static TaskScheduler SyncScheduler { get; private set; }
        
        public static TaskFactory SyncFactory { get; private set; }
    }

class MyViewModel
{
    public void Execute()
    {
        Task.Factory
            .StartNew(() => { })
            .ContinueWith(t => {  }, TaskHelper.SyncScheduler);
    }
}

class MyViewModel
{
    public void Execute()
    {
        TaskHelper.SyncFactory.StartNew(() => { });
    }
}