Try using the marshal method instead.
Dim m_inventorApp As Inventor.Application
Try ' Try to use active inventor instance
Try
m_inventorApp = System.Runtime.InteropServices.Marshal.GetActiveObject("Inventor.Application")
m_inventorApp.SilentOperation = True
Catch ' If not active, create a new instance of Inventor
Dim inventorAppType As Type = System.Type.GetTypeFromProgID("Inventor.Application")
m_inventorApp = System.Activator.CreateInstance(inventorAppType)
' Must set visible explicitly
m_inventorApp.Visible = True
m_inventorApp.SilentOperation = True
End Try
Catch
'Cant get or create an instance of inventor.
End Try