Can't declare the viewmodel in XAML for this code - wpf

Question

I've reviewed as many 'how to declare the viewmodel in XAML' posts I can and still can't figure this out. I'm using the simple program below to learn the basics regarding binding and this code works. When I click to insert(add) items, the listbox automatically reflects that change, as well as when I clear the list.
See my question(s) after the code.
Model
Namespace MVVM3
Public Class MyListItem
Public Property MyListItemID() As Integer
Public Property Name() As String
End Class
End Namespace
ViewModel
Namespace MVVM3
Public Class ViewModel
Implements INotifyPropertyChanged
Public Property allIdeas() As New ObservableCollection(Of MyListItem)
Public Sub New()
For index = 0 To 9
Dim anItem As New MyListItem
anItem.Name = "Idea " & index
allIdeas.Add(anItem)
Next
End Sub
Public Sub InsertAnItem()
Dim anItem As New MyListItem
anItem.Name = "Item " & allIdeas.Count()
allIdeas.Add(anItem)
NotifyPropertyChanged("allIdeas")
End Sub
Public Sub ClearStoredList()
allIdeas.Clear()
NotifyPropertyChanged("allIdeas")
End Sub
Public Event PropertyChanged As PropertyChangedEventHandler _
Implements INotifyPropertyChanged.PropertyChanged
Public Sub NotifyPropertyChanged(ByVal propertyName As String)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
End Sub
End Class
End Namespace
View
<Window x:Class="MVVM3.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:mvvm3"
Title="MainWindow" SizeToContent="WidthAndHeight" WindowStartupLocation="CenterScreen"
>
<StackPanel Grid.Column="1">
<Button Margin="25" Content="Insert an item" Click="InsertAnItem_Click"/>
<Button Margin="25" Content="Clear stored list" Click="ClearStoredList_Click"/>
<ListBox Name="listBox3" ItemsSource="{Binding allIdeas}" DisplayMemberPath="Name" Height="100">
</ListBox>
</StackPanel>
</Window>
Code Behind
Namespace MVVM3
Partial Class MainWindow
Private vm = New ViewModel
Sub New()
InitializeComponent()
DataContext = vm
End Sub
Private Sub InsertAnItem_Click(sender As Object, e As RoutedEventArgs)
vm.InsertAnItem()
End Sub
Private Sub ClearStoredList_Click(sender As Object, e As RoutedEventArgs)
vm.ClearStoredList()
End Sub
End Class
End Namespace
I want to move the ViewModel declaration into XAML to eliminate code behind. If I comment out DataContext = vm, no matter what method I've followed from various posts, the binding no longer updates the listbox.
The following changes result in the listbox showing the initial assignment that takes place in ViewModel.New, but after that no changes are reflected:
<Window x:Class="MVVM3.MainWindow"
...
xmlns:local="clr-namespace:mvvm3"
xmlns:vm="clr-namespace:mvvm3.MVVM3"
...
>
<Window.DataContext>
<vm:ViewModel/>
</Window.DataContext>
What am I missing? Is it a namespace problem?
I'm hoping to move on to Commands and ViewModel locators, but I don't see how I can do that until I understand this.

Answer 1

When you declare an instance of the ViewModel class in XAML, you may access it in code behind by casting the value of the DataContext property to the ViewModel type:
Partial Class MainWindow
Sub New()
InitializeComponent()
End Sub
Private Sub InsertAnItem_Click(sender As Object, e As RoutedEventArgs)
CType(DataContext, ViewModel).InsertAnItem()
End Sub
Private Sub ClearStoredList_Click(sender As Object, e As RoutedEventArgs)
CType(DataContext, ViewModel).ClearStoredList()
End Sub
End Class