C# と VB.NET の質問掲示板

普通に考えると、VMがCLASS::PropertyChangedを拾う形になるんじゃないでしょうか。
class VM {
    public ObservableCollection<CLASS> Items { get; private set; }
    public VM() {
        Items = new ObservableCollection<CLASS>();
        Items.CollectionChanged += this.OnItemsChanged;
    }
    private void OnItemsChanged(object sender, NotifyPCollecctionChangedEventArgs e) {
        if (e.OldItems != null) {
            foreach (CLASS item in e.OldItems) {
                item.PropertyChanged -= this.OnItemChanged;
            }
        }
        if (e.NewItems != null) {
            foreach (CLASS item in e.NewItems) {
                item.PropertyChanged += this.OnItemChanged;
            }
        }
    }
    private void OnItemChanged(object sender, PropertyChangedEventArgs e) {
        CLASS item = (CLASS)sender;
        // todo
    }
}