public RootViewModel(ViewModelBase view)
        {
            CurrentContentVM = view;
        }

        private ViewModelBase currentContentVM;
        public ViewModelBase CurrentContentVM
        {
            get
            {
                return currentContentVM;
            }
            set
            {
                currentContentVM = value;
                RaisePropertyChanged(() => CurrentContentVM);
            }
        }

<Window.Resources>
        <DataTemplate DataType="{x:Type vm:AuthViewModel}">
            <local:AuthControl />
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:CalculateViewModel}">
            <view:CalculateControl />
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:CaloriesViewModel}">
            <view:CaloriesControl/>
        </DataTemplate>
    </Window.Resources>
    <Grid>
        <ContentControl Content="{Binding CurrentContentVM}"/>
    </Grid>

private string _fieldFio;
        RootViewModel root;
        public string FieldFio
        {
            get
            {
                return _fieldFio;
            }
            set
            {
                _fieldFio = value;
                RaisePropertyChanged(() => FieldFio);
            }
        }

        public ICommand CommandLogin //Это команда на кнопке
        {
            get
            {
                return _commandLogin ?? (_commandLogin = new RelayCommand(() => {
                    if (!string.IsNullOrEmpty(FieldFio) && FieldFio.Equals("Admin"))
                        root = new RootViewModel(new CaloriesViewModel());
                    else
                        MessageBox.Show("Неверный формат входной строки");
                }));
            }
        }

public MainWindow()
        {
            InitializeComponent();
            DataContext = new ViewModels.RootViewModel(new ViewModels.AuthViewModel());
        }