Vertical align in StackLayout, which is inside RelativeLayout

I'm using RelativeLayout to create covering background image. I want to place other elements into StackLayout, so I have such code:

<RelativeLayout x:Name="MainLayout">
        <Image Aspect="AspectFill" Source="Images/bg.png"
                        RelativeLayout.WidthConstraint=
                          "{ConstraintExpression Type=RelativeToParent, Property=Width}"
                        RelativeLayout.HeightConstraint=
                          "{ConstraintExpression Type=RelativeToParent, Property=Height}"/>
            <StackLayout BackgroundColor="Gray" VerticalOptions="Center"
                    RelativeLayout.WidthConstraint=
                      "{ConstraintExpression Type=RelativeToParent, Property=Width}"
                    RelativeLayout.HeightConstraint=
                      "{ConstraintExpression Type=RelativeToParent, Property=Height}">

            <Label XAlign="Center" TextColor="White" FontSize="18" BackgroundColor="Purple">La la la:</Label>
            <Grid BackgroundColor="Black">
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto" />
                    <RowDefinition Height="Auto" />
                </Grid.RowDefinitions>

                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>

                <Label BackgroundColor="Red" Grid.Row="0" Grid.Column="0">Red</Label>
                <Label BackgroundColor="Yellow" Grid.Row="1" Grid.Column="0">Red</Label>
                <Label BackgroundColor="Blue" Grid.Row="0" Grid.Column="1">Red</Label>
                <Label BackgroundColor="Green" Grid.Row="1" Grid.Column="1">Red</Label>
            </Grid>
        </StackLayout>
    </RelativeLayout>

In this situation Label and Grid are placed at the top of the screen. When I remove RelativeLayout and Image, it works ok, Label and Grid are placed at the middle. How to fix it?