Proportionately resize entire Visual Basic WPF application + contents - wpf

Question

I have a third party WPF application I am creating for Windows that I need to be resizable.
I have gotten it to resize (proportionally) with the grip but cannot get everything else to work correctly in regards to anchor/layout ect. The buttons would not align correctly with the background no matter what I did.
I'm wondering if there is an easier way to simply resize the whole application and all the contents via the grip. It would look like you would be resizing the photo below.
I have a window, grid (with background) buttons and text. Below is a screenshot of the iOS version of the app. The program will continue to be a 4:3 ratio.

Answer 1

Seems most of this is straightforward WPF, apart from the requirement to keep the buttons square as the window is resized horizontally or vertically. Ideally we would force a horiz resize to force the height, and vice versa, but a bit of googling seems to indicate this is surprisingly difficult to achieve with pro-looking results. So far this xaml is the best I've come up with. Try it with and without the viewbox:
<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" Height="260" Width="260">
<Viewbox Stretch="Uniform">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
<RowDefinition Height="1*"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*"/>
<ColumnDefinition Width="1*"/>
<ColumnDefinition Width="1*"/>
</Grid.ColumnDefinitions>
<Button Content="Button" Margin="4"/>
<Button Content="Button" Grid.Column="1" Margin="4"/>
<Button Content="Button" Grid.Column="2" Margin="4"/>
<Button Content="Button" Grid.Row="1" Margin="4"/>
<Button Content="Button" Grid.Column="1" Grid.Row="1" Margin="4"/>
<Button Content="Button" Grid.Column="2" Grid.Row="1" Margin="4"/>
<Button Content="Button" Grid.Row="2" Margin="4"/>
<Button Content="Button" Grid.Column="1" Grid.Row="2" Margin="4"/>
<Button Content="Button" Grid.Column="2" Grid.Row="2" Margin="4"/>
</Grid>
</Viewbox>