NCARB utilizes a process called "conjunctive scoring" in ARE 4.0 to combine your performance on the multiple-choice section and the graphic vignette section into a single score. In this scoring model, you have the opportunity to compensate for a poor vignette performance if you performed well on the multiple-choice section and vice versa.
So if you are weak at multiple choice, the vignette can help bump up your score, and vice veresa.