How to pass a reference to an object through CommandParameter

I have a MainViewModel class which is the basis for my navigation.

Within that class I have this method with the purpose of changing the parameter object passed in to the selected vendor.

class MainViewModel
{
        public Command ShowVendorDialogCommand
        {
            get;
            private set;
        }
        private void ShowVendorDialog(object parameter)
        {
            if (parameter != null)
            {
                VendorDialog vd = new VendorDialog();
                VendorDialogViewModel vm = new VendorDialogViewModel();
                vd.DataContext = vm;
                vm.PropertyChanged += (s, e) =>
                {
                    if (e.PropertyName == "CloseDialog")
                    {
                        vd.Close();
                    }
                };
                vd.ShowDialog();

                if (vm.DialogResult)
                {
                    parameter = vm.SelectedVendor.Copy() as Vendor;
                }
            }
        }
}

The class that is affected by this method is below:

    class InventoryStyleSingleViewModel
    {
            public Vendor
            { 
                 get
                 {
                      retu _Vendor;
                 }
                 set
                 {
                      if (value != null)
                      {
                           _Vendor = value;
                           OnPropertyChanged("Vendor");
                      }
                 }
            }
            private Vendor _Vendor;

            ........
    }

I am essentially trying to pass the Vendor property as a reference type through the CommandParameter property to the ShowVendorDialog which is executed through a RelayCommand, I am just not sure how to accomplish the reference part.

Here is the xaml that binds to the ShowVendorDialogCommand.

<Button Width="50" DockPanel.Dock="Left" Command="{Binding ElementName=BeginWindow, Path=DataContext.ShowVendorDialogCommand}" CommandParameter="{Binding Vendor}" Content="..." />

This does not accomplish what I need as the Vendor property is passed by value into the ShowVendorDialog function.

Is there anyway to pass the Vendor by reference?