The preview and rendered files are not identical in Vegas 10 and 11. It looks like the amount of noise reduction is less in the rendered file than the preview.
I've gone back to NV 2 which doesn't appear to have this problem.
Also sometimes NV3 does not work at all unless you reapply it.
Could you please report the problem to support [at] neatvideo.com with a small test project allowing to reproduce the issue. Perhaps we have a solution ready.