As said in the comments, it's better to right out edit the vector image in a vector editing application. There is a pretty adequate solution in Photoshop, however. I'm supposing the separate shapes are in a single layer for this workflow:
- Draw a white rectangle on a new layer—preferable using the shape tool, yielding a vector layer as a result;
- Manipulate this rectangle so it is on top of the shapes and has the right position and tilt, partially overlapping the shapes just like your box;
- Change the opacity of the rectangle layer to ~40% and (optionally) change its blending mode to 'Screen';
- Make sure that the rectangle layer is right on top of the layer with the shapes in the Layers palette, then hover over the border between the tow layer entries in the Layers palette while holding Alt / Option; you'll see a cursor consisting of a white square and a cornered arrow downwards.
- Click when you see this arrow to apply the rectangle layer to the shapes layer only.
You might want to group the shape layer and the rectangle layer together so moving one of them around doesn't leave the other behind.
You can play with the colour, the opacity and the blending mode of the rectangle layer to affect the intensity and colour of the lightening effect.