One of the main benefits of working with Vector Paint is that you can clone colors from other layers while you paint. The stroke color is determined by the pixels at the position where you begin to paint.
The Color Clone affects strokes only while you draw and has no effect on a completed stroke. This is the only Brush Setting you can't apply after drawing. Clicking either the Eyedropper or Color swatch turns off the Color Clone feature.
Additionally, try Shift-clicking the Color Swatch to open the Set Value dialog box for Opacity (without turning off Color Clone.)
This works especially well if the layer you are painting is below the layer you are cloning from within the timeline. Drag the layer window out from the comp window tab.