I've been using one of the custom-fit Evolution (3 or 4 layer) covers for a couple of years now. During that time, the VX was parked outside. The Evolution was good for keeping tree sap, bird droppings, and UV rays off the paint, but it wasn't water-proof so washing the VX after it rained seemed like a good idea because dust that had landed on the cover over time would be washed through the material and end up on the paint. Even though the cover was custom fit, it would still move around a little when the wind blew, so leaving dust on the paint (between paint and cover) didn't seem like a wise idea.

The evolution seemed to deteriorate quite a bit over time too. The material just seemed to pull apart, making it thin in spots and more susceptible to tearing.

I think it depends on whether your VX is always outside or not. If it is, something like the Weathershield model might be a better way to go.