WPF

<DockPanel LastChildFill="True">
        <views:Header DockPanel.Dock="Top" />
        <views:Footer DockPanel.Dock="Bottom" />
        <views:Body />
</DockPanel>

<UserControl.DataContext>
    <vm:BodyViewModel />
</UserControl.DataContext>

<DockPanel LastChildFill="True">
    <views:Header DockPanel.Dock="Top" x:Name="viewHeader" />
    <views:Footer DockPanel.Dock="Bottom" x:Name="viewFooter" />
    <views:Body x:Name="viewBody" />
</DockPanel>

public partial class MainWindow : Window
{
    HeaderViewModel Header { get; set; }
    FooterViewModel Footer { get; set; }
    BodyViewModel Body { get; set; }

    public MainWindow()
    {
        InitializeComponent();

        Header = viewHeader.DataContext as HeaderViewModel;
        Footer = viewFooter.DataContext as FooterViewModel;
        Body = viewBody.DataContext as BodyViewModel;
    }
}

class SomeClass
{
    public void AlterHeaderText(HeaderViewModel header)
    {
        // Do some logic with viewmodel.
    }
}

public partial class WizPgQuery : WizardPageBase, IDataErrorInfo
{
    public string this[string columnName]
    {
        get
        {
            string value;
            switch (columnName)
            {
                case "ClassName": value = this.ValidateClassName(); break;
                case "QueryText": value = this.ValidateQueryText(); break;
                default         : value = string.Empty;             break;
            }
            return value;
        }
    }

    public string Error
    {
        get { throw new NotImplementedException(); }
    }

private string className;
public string ClassName
{
    get { return this.className; }
    set
    {
        if (value != this.className)
        {
            this.className = value;
            this.NotifyPropertyChanged();
        }
    }
}

private string ValidateClassName()
{
    string value = (this.ClassName.IsValidClassName())
                    ? ""
                    : "The class name must be a valid C# identifier.";
    // just to make sure it's getting called
    if (!string.IsNullOrEmpty(value)) { SystemSounds.Beep.Play(); }
    return value;
}

<TextBox x:Name="textboxClassName" Grid.Column="1" 
         Text="{Binding Path=ClassName, 
         Mode=TwoWay, 
         ValidatesOnDataErrors=True, 
         ValidatesOnExceptions=True, 
         NotifyOnValidationError=True,
         UdateSourceTrigger=PropertyChanged}"
         TextChanged="TextboxClassName_TextChanged" 
         ToolTip="Base tool tip" />