Like Kat & Bob said, try to see if the high pressure spray at the car wash will remove at least part of of (I've had good luck with that, too.)

Before you spend any more money, you might try some household cleaners. It may require a little muscle power (as long as it's coming off - even slowly - you're moving in the right direction.)

VX specific...

I would up with a thick glob of tar on my cladding (the textured part below the rear marker where you can see it.) I used plain ol' Bug & Tar Remover to get rid of it and didn't have any discoloring, residual film or change in the texture. Yours should be easier since it's on the smooth part of the cladding. It was a slow process and required a LOT of elbow grease. I mention this product because I've also had good luck using the same stuff to remove road paint from other vehicles.

I also inherited a horrible product meant to darken the cladding called Bondo Restore Black (I DO NOT recommend!) when I bought my VX. I used SuperClean Cleaner/Degreaser to remove it. BE CAREFUL because it's some evil stuff. I accidentally sprayed a few drops on my fuel fill door and it discolored the paint. DO NOT use withing WEARING GLOVES gloves. By the time I was finished my hands looked like hamburger. I used it full strength on my cladding and it didn't harm it at all. In fact, it looked great! That left me with the impression that our cladding is some pretty hearty stuff!

But, it's always best to start with the wimpiest chemicals and only work your way up to the heavier stuff only if needed.