Is the xjr superior to the xj8 vanden plas?

You are comparing apples and oranges. The XJR has more power (supercharged) and a bit stiffer suspension. The Vanden Plas a softer more refined ride and slightly less horsepower (normal aspirated)
Both are fine luxury saloons. Since you didn't mention years it is difficult to determine what you are comparing.

Vern covered it. Tell us more: do you have two cars you are comparing or are you about to start searching and want to know what to spend your time on? R has a better transmission, but if the VP is an 03, probably doesn't matter. Both models have some common problems depending on year and mileage.

My VDP is SC'd, so I have the best of both worlds.

Yo Highhorse! My 2001 VDP is also S/C. Is it superior to a 2001 XJR? Maybe, maybe not. In '01, the sticker on the VDP S/C was more than $15k greater than the XJR. I don't use the picnic tables, rear passenger front seat controls, or for that matter the back seat very much. Also, the "old man" wheels are not sexy, at all. But,,, I do love all that chrome and bragging rights (total of 234 made in 2001.)

Superior is so subjective.

If you want the overall fastest and best handling; get an XJR with the fancy brake package. If you want the cheapest entry price, get a SWB XJ8. If you want the affordable long wheelbase with all the toys, get a VDP. Want all the toys and plenty of power, get the VDP SC. Want the looks of the XJR but the base drivetrain, get an XJ8 Sport.

Even the "worst" XJ8 is an enjoyable car to drive.