MVVM Экранная клавиатура и ввод в разные TextBox

Так как задача специфична, без код-бихайнда обойтись не удалось, но удалось реализовать то, что нужно. Я сделал в рамках обычного окна, но вы можете распределить логику по юзер-контролам, это не должно составить труда.

Я не использую внешние библиотеки и классы для MVVM реализаций, вместо этого использую 2 следующих вспомогательных класса, думаю, вы знаете, что они делают:

public class NotifyPropertyChanged : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

public class RelayCommand : ICommand
{
    private readonly Action<object> _execute;
    private readonly Predicate<object> _canExecute;

    public event EventHandler CanExecuteChanged
    {
        add => CommandManager.RequerySuggested += value;
        remove => CommandManager.RequerySuggested -= value;
    }

    public RelayCommand(Action<object> execute, Predicate<object> canExecute = null)
        => (_execute, _canExecute) = (execute, canExecute);

    public bool CanExecute(object parameter)
        => _canExecute == null || _canExecute(parameter);

    public void Execute(object parameter)
        => _execute(parameter);
}

MainViewModel.cs

public class MainViewModel : NotifyPropertyChanged
{
    private const string _deleteText = "Удалить";
    private const string _clearText = "Очистить";
    private const int _loginLength = 6;
    private const int _passwordLength = 4;

    private string _uid = string.Empty;
    private string _password = string.Empty;
    private ICommand _keyPressCommand;
    private ICommand _loginCommand;

    public bool LoginFocused { get; set; }
    public bool PasswordFocused { get; set; }

    public string Uid
    {
        get => _uid;
        set
        {
            _uid = value;
            OnPropertyChanged();
        }
    }

    public string Password
    {
        get => _password;
        set
        {
            _password = value;
            OnPropertyChanged();
            OnPropertyChanged(nameof(PasswordView));
        }
    }

    public string PasswordView => new string('\u25cf', _password?.Length ?? 0);

    public ICommand KeyPressCommand => _keyPressCommand ?? (_keyPressCommand = new RelayCommand(parameter =>
    {
        if (parameter is int number)
        {
            if (LoginFocused)
                Uid += number;
            else if (PasswordFocused)
                Password += number;
        }
        if (parameter is string command)
        {
            switch (command)
            {
                case _deleteText:
                    if (LoginFocused)
                        Uid = Uid.Remove(Uid.Length - 1);
                    else if (PasswordFocused)
                        Password = Password.Remove(Password.Length - 1);
                    break;
                case _clearText:
                    if (LoginFocused)
                        Uid = string.Empty;
                    else if (PasswordFocused)
                        Password = string.Empty;
                    break;
            }
        }
    }, parameter =>
        (parameter is int && ((LoginFocused && Uid?.Length < _loginLength) || (PasswordFocused && Password?.Length < _passwordLength))) ||
        (parameter is string && ((LoginFocused && Uid?.Length > 0) || (PasswordFocused && Password?.Length > 0)))
    ));

    public ICommand LoginCommand => _loginCommand ?? (_loginCommand = new RelayCommand(parameter =>
    {
        MessageBox.Show(string.Format("Uid: {0}{1}Password: {2}", Uid, Environment.NewLine, Password));
    }));

    public IEnumerable<object> PadKeys => GetKeys();

    private IEnumerable<object> GetKeys()
    {
        for (int i = 1; i < 10; i++)
            yield return i;
        yield return _deleteText;
        yield return 0;
        yield return _clearText;
    }

    public MainViewModel()
    {
    }
}

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    private readonly MainViewModel _vm;

    public MainWindow()
    {
        InitializeComponent();
        _vm = new MainViewModel();
        DataContext = _vm;
    }

    private void LoginBox_FocusChanged(object sender, RoutedEventArgs e)
    {
        if (sender is TextBox textBox)
            _vm.LoginFocused = textBox.IsFocused;
    }

    private void PasswordBox_FocusChanged(object sender, RoutedEventArgs e)
    {
        if (sender is TextBox textBox)
            _vm.PasswordFocused = textBox.IsFocused;
    }
}

Разметка адаптивная, можно менять размер окна, размер кнопок подстроится под любое разрешение. Настройте себе марджины и паддинги по вкусу.

MainWindow.xaml

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Width="600" Height="300" FontSize="20" FocusManager.FocusedElement="{Binding ElementName=LoginTextBox}">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <StackPanel Orientation="Vertical" Margin="5">
            <TextBlock Text="Номер пользователя:" Margin="0,5,0,0"/>
            <TextBox x:Name="LoginTextBox" Margin="0,5" Text="{Binding Uid, Mode=OneWay}" IsReadOnly="True" GotFocus="LoginBox_FocusChanged" LostFocus="LoginBox_FocusChanged"/>
            <TextBlock Text="Пароль:" Margin="0,5,0,0" />
            <TextBox Margin="0,5" Text="{Binding PasswordView, Mode=OneWay}" IsReadOnly="True" GotFocus="PasswordBox_FocusChanged" LostFocus="PasswordBox_FocusChanged"/>
            <Button Content="Войти" Padding="20,5" HorizontalAlignment="Right" Command="{Binding LoginCommand}"/>
        </StackPanel>
        <ItemsControl ItemsSource="{Binding PadKeys}" Grid.Column="1">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Button Margin="5" Command="{Binding DataContext.KeyPressCommand, RelativeSource={RelativeSource AncestorType=Window}}" CommandParameter="{Binding}" Focusable="False" >
                        <Button.Content>
                            <Viewbox Margin="5">
                                <TextBlock Text="{Binding}"/>
                            </Viewbox>
                        </Button.Content>
                    </Button>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <UniformGrid Columns="3"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
    </Grid>
</Window>