tamron 17-50mm f2.8 vc

just bought this lens yesterday as i wanted a faster standard zoom, partly as i want to start using filters again, incl a circular polariser - my nikkor 18-70mm is just too slow for that. had little time to play but first impressions are good - a bit plasticky but no more so than my nikkor 18-70 (in fact the tamron seems more solid). the vc system seems to work well too.

as i said i want to start using filters, a polariser and i'm also keen to experiment with one of the new 10 stop nd filters. the tamron has a 72mm thread and i would like to use the same screw in filters on my sigma 10-20mm, which has a 77mm thread (the new sigma 17-50 os has a 77 thread but i could see no reason to pay £300 more for a lens which appears to offer no advantage over the tamron).

so my question for existing tamron 17-50mm vc users is; if i mount a 77mm screw in filter on the 17-50 using a 77-72mm step up ring, will i experience any significant vignetting at the wide end of the zoom range (far cheaper to buy 72 & 77mm polarisers than buy the sigma 17-50).

a bit of a ramble, hope it makes sense.