VBCS is a visual development environment that allows you to develop JET applications faster.
When you switch the visual view of a page to the "code" view you'll see Oracle JET code that you can modify directly.
You should be able to pick up any JET code/component and bring them into a VBCS app.
In addition you can pick up Oracle JET composite components and add them to the VBCS visual editor.
VBCS adds some abstraction layers around the JET toolkit making it easier to define actions, REST calls etc.
So at the end of the day with both your are creating an Oracle JET application, the question is whether you prefer a more visual way to develop - which is what VBCS gives you - or whether you rather code everything directly.
(One more difference VBCS apps can only run in the Oracle Cloud - JET apps can be deployed anywhere).