Strangely, the last reason I'd use a UV filter is for filtering out ultraviolet light.

They WILL, but their most important use is for lens protection. Some of the most heart-wrenching sights I have ever seen are the used top-flight Leica and Hasselblad lenses with a definite, dull center spots caused by constant knee-jerk cleaning. I do not care what method is used to clean a lens, they will all, without exception, wear on the coating, and eventually the combined effect will be image degredation ... mainly manifested by a reduction in CONTRAST - well before defintion.

I KEEP UV filters on all my lenses. I can clean those to my hearts content, and replace them if and when I've ground the centers to transluscence (hasn't happened to any of them yet). I clean the lenses themselves, on the average, about once every two years.
I have a number of UV filters, B&W, Hoya, Tiffen, Heliopan, Singh-Ray, and "unidentified from the bargain boxes". I cannot tell one whit of difference between any of them - or, for that matter, photogrpaphs taken without them.

Vignetting can only be caused by a filter too small to cover the optical field, not by some alchemic, mysterious propery of the filter itself... in the same manner as a too-long lens shade used on a wide-angle lens.