WPF project within another WPF project. Commands not working - wpf

Question

I have a project solution that contains 2 WPF projects. Project B is contained, and called, from within project A.
The issue I'm having, is that the button commands on project B MainWindow are not hitting it's corresponding ViewModel. This doesn't happen when I run Project B as a standalone application (everything's fine).
In both Projects A and B, I'm using Galasofts MVVM light framework, housing my view models in the static ViewModelLocator class.
Any idea's would be greatly appreciated.
Thanks
Edit: For simplicity, I've just included 1 button.
ribbon:RibbonControl xmlns:Designer="clr-namespace:Nouvem.LabelDesigner.View.Designer" x:Class="Nouvem.LabelDesigner.View.Designer.DesignerView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:ribbon="clr-namespace:Microsoft.Windows.Controls.Ribbon;assembly=RibbonControlsLibrary"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:command="http://www.galasoft.ch/mvvmlight"
xmlns:sys="clr-namespace:System;assembly=mscorlib"
xmlns:systemMessage="clr-namespace:Nouvem.LabelDesigner.View.SystemMessage"
xmlns:loc="clr-namespace:Nouvem.Shared.Localisation;assembly=Nouvem.Shared"
UseLayoutRounding="True"
mc:Ignorable="d"
d:DesignHeight="1000" d:DesignWidth="1200"
Background="White"
DataContext="{Binding LabelDesigner, Source={StaticResource Locator}}" >
<Grid>
<Button Command="{Binding TestCommand}"/>
</Grid>
</ribbon:RibbonControl>
ViewModel
public RelayCommand TestCommand { get; set; }
public LabelDesignerViewModel()
{
if (this.IsInDesignMode)
{
return;
}
this.TestCommand = new RelayCommand(() =>
{
// test - not hitting when called from another project
});
}
`

Answer 1

if you do something like new ProjectB.MainWindow.Show() in Project A, then all logic in Project B/App.xaml.cs and resources in Project B/App.xaml are ignored.
if you run Project A, then Application.Current is ProjectA.App and ProjectB.App is not instantiated.
there are two solutions:
1. Create Project B as library:
create new shared library called Project B Library, which you will reference from Project A as well as from Project B. Project B will be just an exe container.
2. Start Project B as new Process
use Process.Start() method to start Project B. You can interact with Project B trought WCF using netNamedPipeBinding