Error de ruta de BindingExpression para la propiedad en ViewModel

I have the below visibility binding that is throwing:

System.Windows.Data Error: 40 : BindingExpression path error: 'Vis' property not found on 'object' ''LoginViewModel' (HashCode=22943289)'. BindingExpression:Path=Vis; DataItem='LoginViewModel' (HashCode=22943289); target element is 'Login' (Name=''); target property is 'Visibility' (type 'Visibility')

Can't see what I've done wrong, the property does exist in the MainViewModel. Maybe I'm going about showing and hiding this the wrong way.

<Window x:Class="Bt.MainWindow"
        xmlns:vm="clr-namespace:Bt"
        xmlns:ctrls="clr-namespace:Bt.Controls">

    <Window.DataContext>
        <vm:MainViewModel x:Name="MWin" />
    </Window.DataContext>

    <Grid>
        <ctrls:Login Visibility="{Binding Vis}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"></ctrls:Login>
    </Grid>
</Window>

Ver modelo:

namespace Bt
{
    class MainViewModel : INotifyPropertyChanged
    {
        public MainViewModel()
        {
            Vis = "Collapsed";
        }

        private string _vis = "Collapsed";
        public string Vis
        {
            get { return _vis; }
            set
            {
                    _vis = value;
                    RaisePropertyChanged("Vis");
            }
        }
    }
}

[EDITAR] Capturing inside the User Control, when the User Control's visibility is changed in the Main window.

I realize that the converter is not being called correctly, so may need some help there as well. As for the rest hopefully you can see what I'm trying to achieve.

Vista

<UserControl x:Class="Bt.Controls.Login" 
             xmlns:app="clr-namespace:Bt" 
             xmlns:viewmodel="clr-namespace:Bt.Controls"
             mc:Ignorable="d"
             Visibility="{Binding Visi,Converter={StaticResource BooleanToVisibilityConverter}}"
            >
</UserControl>

Ver modelo:

namespace Bt.Controls
{
    class LoginViewModel : INotifyPropertyChanged
    {
        public LoginViewModel(){}

        private bool _visi = true;
        public bool Visi
        {
            get { return _visi; }
            set
            {
                _visi = value;
                RaisePropertyChanged("Visi");
                MessageBox.Show("Visi set");
                reset_timer(_visi);
            }
        }
    }

    [ValueConversion(typeof(bool), typeof(Visibility))]
    public class VisibilityConverter : IValueConverter
    {
        public const string Invert = "Invert";

        #region IValueConverter Members

        public object Convert(object value, Type targetType, object parameter,
            System.Globalization.CultureInfo culture)
        {
            if (targetType != typeof(Visibility))
                throw new InvalidOperationException("The target must be a Visibility.");

            bool? bValue = (bool?)value;

            if (parameter != null && parameter as string == Invert)
                bValue = !bValue;

            return bValue.HasValue && bValue.Value ? Visibility.Visible : Visibility.Collapsed;
        }

        public object ConvertBack(object value, Type targetType, object parameter,
            System.Globalization.CultureInfo culture)
        {
            return ((value is Visibility) && (((Visibility)value) == Visibility.Visible));
        }
        #endregion
    }
}