I bet the bridge is made of some sort of MDF partical board, meant only to support knick-knacks and the like. The VP180 is one hell of a knick-knack, indeed.
I think the cheapest, easiest and probably best solution would be to go to your local hardware store (Home Depot or whatever) and buy a piece of solid, unpainted wood, 1/2 thick should do, that spans the gap. Just laying this across the span should reinforce the span enough to prevent sagging.
You could probably even hide the piece of wood by moving it rearward on the shelf so that only the back of the speaker lays on it. In this configuration, the VP180 will be pointing more downward, as a result of its tail being placed higher in the air, on the piece of reinforcement wood. I would then insert a couple of short screws into the back of the speaker cabinet (careful to avoid going too deep into the speaker cavity and causing havoc), and run some picture hanging wire from those screws, to attach to the shelf to prevent a sliding disaster.
