Vignetting occurs when something gets in the way of the view of the lens. It can occur when anything (especially when I see new photogs nowadays shooting with their left hand over the lens and their pinkies sticking out.) gets in front of the lens in the view angle of the lens in use and is not confined to cylindrical lens hoods.
The petal/flower shaped hoods (that most manufacturers dedicate to a particular lens) have cutouts on the four corners and a fixed orientation that help to minimize the chances of vignetting while maximizing the shading of the front of the lens.
It still doesnt help if your fingers get ahead of the hood. In a pinch, with SLR's, you can use your hand on the side that the sun is on and shade the lens. (YMMV)
oh I see, but won't that mean the petal shaped hood then would have "holes" at the corners which while may be good for preventing vignetting, but won't help to prevent flare esp if the sun or any light happens to be at the corners?