CustomControls are never done with mvvm.
What you want is a reusable view(user control) of your data and not a control(custom control).
UserControls and CustomControls are two completely different beasts.
EDIT:
Notwithstanding why UserControls were originally developed, in MVVM typically you use a UserControl when you want a reuseable view which is specific to your model/viewmodel. Its just XAMl without any code behind (except for the auto generated InitializeComponent stuff). Generally you keep a UserControl in the same project that you use it in.
You go for a CustomControl when you want a generic piece of functionality which requires a view and which has potential use even outside the scope of your current application. Here the control is actually defined in a code file and the look (which can be overriden) comes via XAML in a resource dictionary.
Generally you keep a CustomControl in a a seperate ControlLibrary project and reference the library in the project you wish to use it in.
With due respect to WallStreetProgrammer, choosing between a user control and a custom control based solely on whether or not you want a lookless control is a bit naive.
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