MVVMLight: Having a remote cs file call the listbox control

Question

New to WPF (especially with MVVMlight).
I have a Windows Form project I want to rebuild in MVVM.
Have the Mainwindow.xaml with a button and a Listbox controls.
I have the MainwindowViewModel.cs file but and working so it can handle simple text update.
I have a new .cs file that has my main code which traverses a folder, subfolder and get file data.
I want to have the remote .cs file update the Listbox control continuously when the file collection is running.
Not sure how to do this.
Not clear on how to pass the collection result to the listbox and do I need a threading approach so the Listbox is immediately updated? Meaning that the main windows is not frozen. Want continous streaming of data.