One way: create a polygon mask. Click the FX icon on the clip, then small green + sign - second from the left in the toolbar. Draw the mask around the area to be shaded.
Then click the larger far-left green + and choose blur, diffuse, or exposure. This will affect only the area within the mask. Make adjustments with the slider in the effects window.
It might look similar to this, which blended the reflection a bit more into the background, using the exposure effect.
View this tutorial. Slightly older VP version is shown but the procedure is the same.
