您正在使用的EventTrigger不支持附加事件。您必须实现自己的custom event trigger，或者您可以简单地从窗口中的Window_Select事件处理程序调用该命令：

var viewModel = DataContext as YourViewModel;
if (viewModel != null)
    viewModel.ElipseSelectedCommand.Execute(null);

这不会以任何方式破坏MVVM模式，因为与使用交互触发器相比，您是从相同的视图调用相同的命令。MVVM不是要从视图中删除代码-而是要分离关注点。

自定义事件触发器起作用了。原问题中的MRE被更新了。下面是相关的更新代码。

<Window x:Class="WpfApp5.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp5"
        xmlns:b="http://schemas.microsoft.com/xaml/behaviors"
        mc:Ignorable="d"
        
        Title="MainWindow" Height="450" Width="800">
    
    <b:Interaction.Triggers>
        <local:RoutedEventTrigger RoutedEvent="local:ElipseControl.Select" >
            <b:InvokeCommandAction Command="{Binding ElipseSelectedCommand}" PassEventArgsToCommand="True" />
        </local:RoutedEventTrigger>
    </b:Interaction.Triggers>
    
    <Grid>
        <ItemsControl ItemsSource="{Binding Elipses}" />
    </Grid>
</Window>

根据custom event trigger，略有更新的版本：

public class RoutedEventTrigger : EventTriggerBase<DependencyObject>
    {
        private RoutedEventHandler _eventHandler;

        public RoutedEvent RoutedEvent { get; set; }

        public RoutedEventTrigger()
        {

        }

        protected override void OnAttached()
        {
            var associatedElement = GetAssociatedElement();

            if (associatedElement != null && RoutedEvent != null)
            {
                _eventHandler = new RoutedEventHandler(OnRoutedEvent);

                associatedElement.AddHandler(RoutedEvent, _eventHandler);
            }
        }

        protected override void OnDetaching()
        {
            var associatedElement = GetAssociatedElement();

            if (associatedElement != null && RoutedEvent != null && _eventHandler != null)
            {
                associatedElement.RemoveHandler(RoutedEvent, _eventHandler);
            }

            base.OnDetaching();
        }

        private FrameworkElement GetAssociatedElement()
        {
            FrameworkElement associatedElement = null;

            if (AssociatedObject is Behavior behavior)
            {
                associatedElement = ((IAttachedObject)behavior).AssociatedObject as FrameworkElement;
            }
            else if (AssociatedObject is FrameworkElement frameworkElement)
            {
                associatedElement = frameworkElement;
            }

            if (associatedElement == null)
            {
                throw new ArgumentException("Routed Event Trigger can only be associated to framework elements");
            }

            return associatedElement;
        }

        private void OnRoutedEvent(object sender, RoutedEventArgs e) => base.OnEvent(e);

        protected override string GetEventName() => RoutedEvent?.Name ?? string.Empty;
    }