A step up ring should not cause vignetting as you are fitting a bigger filter onto a smaller lens. Going the other way round would be bad news.
While it sounds like a good idea, personally I don't like it as the filter often gets stuck to the step up ring and they can be a pain to separate again. Also, while you have them connected you can't put the original lens cap back on. You'll either have to remove the step up or buy a bigger lens cap too. Similarly there will be a problem with fitting a hood. Actually, on a wide angle the hood is pretty much useless for flare prevention anyway so it's not great loss.
Don't forget that on a wide angle, a CPL will likely cause an uneven effect through the image due to the effect varying with angle relative to the light source.
If you want to further stack a filter holder on the front of that, do remember the step up ring and CPL will push the holder outwards from the lens. It will therefore start to intrude on the image earlier than if they weren't there, so you may have a less wide usable angle than manufacturer's specification.
I think Cokin do a CPL for their system too, which can be rotated independently of the rectangular filters, so if you are thinking of going that route you might consider using only that.
_________________Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L, 100L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
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