Acetic acid for weeds?

Does anyone out there have any knowledge of how acetic acid (concentrated form) is diluted with what to kill weeds? I used a product a couple years ago that was comprised mainly of acetic acid and worked like a charm on weeds (on a hot day). I've been asked to spray RoundUp on a client's gravel driveway but I have reservations about using that particular product. We are able to procure acetic acid in bulk but am hoping and praying that one of you wizards out there know how to mix it and with what. Also some testimonials pro and con would be greatly appreciated.