Thanks to
@RichardDeeming, I got my final solution:
<DataGrid Tag="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:ToolPathPage}},Path=DataContext.(viewModel:ToolPathPageViewModel.Clear_AllCommand)}">
<DataGrid.ContextMenu>
<ContextMenu>
<MenuItem Header="Clear" Command="{Binding PlacementTarget.Tag,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=ContextMenu}}"/>
</ContextMenu>
</DataGrid.ContextMenu>
</DataGrid>