Nikon 18-200 lens and Vignette

[johnarush]

I have read that some folks have not had many issues with vignetting with Nikon's current 18-200 VR lens; however, I have found that using a B and W 77E circular polarizing filter creates vignetting at about 20 mm. I am using the 77 on a 72 to 77 step up ring. Is this causing the problem? Would a 72 mm polarizer directly on the lens vignette less? I realize they make slim polarizers but I tend not to like that option as they lack front threads. I also have a large 95 mm filter (remnant of shooting with pentax 67 zoom lens). Would using a step up with this filter eliminate vignetting on the 18-200?
Thank you,
J

[I CHNGE]

This link may help answer some of your questions...

http://www.kenrockwell.com/nikon/18200/18200-filters.htm

quoted from the link...

"...I use a stepping ring to change the 72 mm thread up to the pro standard of 77 mm. I use a professional B+W...OK, with my thick polarizer and step up ring I get just a hint of darkening at the farthest corners at 18mm. If I come just a little off the 18mm stop it goes away. I'm not going to worry about it..."