Vividness means clarity or brightness, like the vividness of a specific, distinct childhood memory or the vividness of a glowing neon sign on a clear night.
When something is vivid, it's brilliant, powerful, or sharp, and the quality of being vivid is vividness. If you go back to the house where you lived as a small child, you might be surprised at the vividness of your recollections. And you may be equally surprised at how quickly the vividness of a crazy dream starts to fade once you wake up. More literal is the vividness of something glowing, bright, or colorful.