<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" x:Class="Simple.Modules.A.MainView"
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"    
    d:DesignWidth="309" d:DesignHeight="246">
    
	<StackPanel>
		<sdk:Label Content="Логин"/>
        <TextBox TextWrapping="Wrap" Text="{Binding LoginText, Mode=TwoWay}"/>
		<sdk:Label Content="Пароль"/>
        <TextBox TextWrapping="Wrap" Text="{Binding PasswordText, Mode=TwoWay}" TextChanged="TextBox_TextChanged" Name="TextBox1" />
        <Button Content="{Binding ButtonContent}" Margin="0,8,0,0">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="MouseMove">
                    <i:InvokeCommandAction Command="{Binding MessageBoxCommand}" CommandParameter="{Binding RelativeSource={RelativeSource Self}, Path=InvokeParameter}">
                    </i:InvokeCommandAction>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </Button>
	</StackPanel>
</UserControl>

namespace Simple.Modules.A
{
    /// <summary>
    /// Interaction logic for DefaultViewA.xaml
    /// </summary>
    public partial class MainView : UserControl
    {
        public MainView(MainViewModel model)
        {
            InitializeComponent();
            this.Loaded += (s, e) =>
                               {
                                   this.DataContext = model;
                               };
        }

    }
}

namespace Simple.Modules.A
{
    public class MainViewModel : ViewModel
    {
        public MainViewModel()
        {
            this.messageBoxCommand = new CommandModel(OnMessageBoxCommand);       
        }
        private string _loginText = "";
        public string LoginText
        {
            get { return _loginText; }
            set { _loginText = value; }
        }

        private string _passwordText = "";
        public string PasswordText
        {
            get { return _passwordText; }
            set { _passwordText = value; }
        }

        private string _buttonContent = "СуперВход";
        public string ButtonContent 
        {
            get { return _buttonContent; }
        }

        private ICommand messageBoxCommand;
        public ICommand MessageBoxCommand
        {
            get { return this.messageBoxCommand; }
        }
        private void OnMessageBoxCommand(object parameter)
        {
            LoginText += "1";
            this.InvokePropertyChanged("LoginText");
        }

    }
}

можно ловить notification от биндинга

 public abstract class ViewModel : INotifyPropertyChanged
    {
        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        protected void InvokePropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }

        #endregion
    }

    public partial class MainView : UserControl
    {
        public MainView(MainViewModel model)
        {
            InitializeComponent();
            this.Loaded += (s, e) =>
                               {
                                   this.DataContext = model;
                                   model.PropertyChanged += OnPropertyChanged;
                               };
        }
        protected void OnPropertyChanged(Object obj, PropertyChangedEventArgs args)
        {
            string s = args.ToString();
        }
    }