It's been awhile since I've shot wide angle stuff and I can see the vignetting was most likely caused by stacking the polarizer on top of the skylight filter but what the heck caused the vast difference in the sky color from right to left?

Thanks guys!! Kevin in CT
"And on the brave and crazy wings of youth
They went flying around in the rain
And their feathers, once so fine, grew torn and tattered
And in the end they traded their tired wings
For the resignation that living brings'
"Before The Deluge" by Jackson Browne