I use the vector method. When I get a bitmap that I think might be good to use as the distressing I bring it in as a bitmap then do an "image Trace", ungroup the resulting trace and remove the white rectangle from the back. Group what I have left and place it over the area you want to distress. Normally I only use this on either the background or the foreground.
The reason I use vector shapes is that if you're doing T shirts using separations you can get funny effects just with "fractal clouds" with transparencies when you export it.
Here how they do it in Illustrator http://retrovectors.com/tutorials/ad...e-illustrator/