MainVm.cs

using System.Collections.ObjectModel;
using System.ComponentModel;
using WpfApplication14.Annotations;

namespace WpfApplication14
{
    public class MainVm : INotifyPropertyChanged
    {
        #region Singleton Setup

        private static readonly MainVm Instance = new MainVm();

        public static MainVm Current
        {
            get { return Instance; }
        }

        #endregion

        #region GUI Vars

        public ObservableCollection<MenuVm> MenuVms { get; set; }

        private MainVm _activeVm;

        public MainVm ActiveVm
        {
            get { return _activeVm; }
            set
            {
                _activeVm = value;
                OnPropertyChanged("ActiveVm");
            }
        }

        #endregion

        #region C'Tor

        public MainVm()
        {
            MenuVms = new ObservableCollection<MenuVm>();
            MenuVms.Add(new MenuVm());
            MenuVms.Add(new MenuVm());
            MenuVms.Add(new MenuVm());
            MenuVms.Add(new MenuVm());
            MenuVms.Add(new MenuVm());
        }

        #endregion

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

MainWindow.cs

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            MainVm.Current.ActiveVm = MainVm.Current; // DOESNT WORK
            //DataContext = MainVm.Current; // WORKS
        }
    }

MainWindow.Xaml

<Window x:Class="WpfApplication14.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>

        <Grid.Resources>
            <CollectionViewSource x:Key="MenuVms" Source="{Binding MenuVms}" />
        </Grid.Resources>
        
        <Button DataContext="{Binding ActiveVm}" Width="25" Height="25" Name="OptionsButton">

            <Button.ContextMenu>
                <ContextMenu Placement="Top">

                    <ContextMenu.ItemsSource>
                        <CompositeCollection>
                            <CollectionContainer Collection="{Binding Source={StaticResource MenuVms}}" />
                        </CompositeCollection>
                    </ContextMenu.ItemsSource>

                </ContextMenu>
            </Button.ContextMenu>

            <!--<Button.Template>
                <ControlTemplate>-->
            <Rectangle Width="25" Height="25" Fill="Pink" />
            <!--</ControlTemplate>
            </Button.Template>-->

        </Button>

    </Grid>
</Window>