Do NOT use salt. Vinegar is not really appropriate either (better for spot treatment than large areas, will not necessarily work in 1 shot, and has no residual action).
Is it just under the rocks bordering the beds that you're having problems? Remove the rocks, remove the weeds by hand, then (ideally) replace the rocks with something else like cedar logs or landscape ties. Then use a combination of hand-weeding and a weed-whacker (=string trimmer). If you want to keep weeds/grasses away from the base of the beds (that is, don't want grass growing up to the logs/ties/rocks/whatnot), then remove the weeds and grass from the area and put down a narrow strip of used carpeting you pick up from the curb on trash day, and cover it with mulch. It will still require periodic hand-weeding and you may have trouble with the lawnmower chewing at the edge of it, though, so frankly I'd recommend making it so you can just run a weedwhacker along the edging material itself and be done with it, much faster.
If you insist on using rocks for edging, you will simply have to roll each rock away a coupla times a year to get at the roots of weeds that have crawled under them; rocks are really not the most practical edging for raised beds. The roots of weeds will crawl under other things (logs, ties, etc) as well, but not nearly as much because there are no gaps like the gaps between rocks, and it is MUCH easier to tip one log or tie out to weed under it than to do it with a dozen rocks
Good luck,
Pat, who admits to having rocks around the flowerbed nearest the driveway, because they're the most functional edging there (car- and snowshovel proof, and will curve around the corner) despite the weeding issues
