Has anyone had any success overlaying the new vector paint in Sculpt Mode across an existing image texture? The vector paint is only showing in the basic viewport but not rendering. Thanks in advance.
You seem to be asking two things here: how to paint over an existing texture, and how to get the painting in the final image. It all comes down to the material of your object:
Sclupt mode paints in an object Color attribute, which you can bring into your shader using the Attribute node as follow:
Having an image is also done using nodes, an Image Texture for your image, and a Mix RGB with your favorite blend type and factor; I used Mix with the color attribute's value as factor but you can do anything you like: