ContextMenuService.Placement doesn't work in setter for MVVM pattern

Question

I have gone through some of the previous similar questions, but am still having issues with the ContextMenuService.Placement. Please suggest.

It worked fine when I followed this approach

<Button x:Name="btn" Content="button"  Click="b_Click" >
        <Button.ContextMenu >
            <ContextMenu   >
                <MenuItem Header="Open" Command="{Binding OnOpen}" ></MenuItem>
                <MenuItem Header="Close" Command="{Binding OnClose}"></MenuItem>                    
            </ContextMenu>
        </Button.ContextMenu>
    </Button>

    private void be_Click(object sender, RoutedEventArgs e)
            {
            btn.ContextMenu.PlacementTarget = btn;//Like this
            btn.ContextMenu.Placement = PlacementMode.Top;//Like this
            btn.ContextMenu.DataContext = btn.DataContext;
            btn.ContextMenu.IsOpen = true;            
            }

But it doesn't work with MVVM pattern.

<Button x:Name="btn" VerticalAlignment="Bottom" ContextMenuService.IsEnabled="False">
    <Button.Style>
        <Style TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
            <Style.Triggers>
                <EventTrigger RoutedEvent="Click">
                    <EventTrigger.Actions>
                        <BeginStoryboard>
                            <Storyboard>
                                <BooleanAnimationUsingKeyFrames Storyboard.TargetProperty="ContextMenu.IsOpen">
                                    <DiscreteBooleanKeyFrame KeyTime="0:0:0" Value="True"/>
                                </BooleanAnimationUsingKeyFrames>
                                <BooleanAnimationUsingKeyFrames Storyboard.TargetProperty="ContextMenu.IsEnabled">
                                    <DiscreteBooleanKeyFrame KeyTime="0:0:0" Value="True"/>
                                </BooleanAnimationUsingKeyFrames>
                            </Storyboard>
                        </BeginStoryboard>
                    </EventTrigger.Actions>
                </EventTrigger>
            </Style.Triggers>
            <Setter Property="ContextMenu">
                <Setter.Value>
                    <ContextMenu Name="AddReportContextMenu" HorizontalAlignment="Right">
                         <MenuItem Header="Open" Command="{Binding OnOpen}" ></MenuItem>
                <MenuItem Header="Close" Command="{Binding OnClose}"></MenuItem>                    
                    </ContextMenu>
                </Setter.Value>
            </Setter>
        </Style>
    </Button.Style>
    </Button>

I tried adding it in setter like this

<Setter Property="ContextMenu.PlacementTarget" Value="{Binding ElementName=btn}"></Setter>
<Setter Property="ContextMenu.Placement" Value="Top"></Setter>

Or Like this:

<Setter Property="ContextMenuService.Placement" Value="Top"></Setter>

The placement property worked well with right click like this if ContextMenuService.IsEnabled is not set it false. I had to set it false to make sure that the ContextMenu only worked with Left Click.

<Button x:Name="btn" ContextMenuService.Placement="Top">

Thank you!