I couldn't find 20% vinegar in town (I didn't look too hard, time contraints and all) so I bought it off Amazon through AG Organics. I got it fast and it was as cheap as I could find on the web. My only complaint was I had to pay a lot in shipping. I think I will buy it in bulk next time, just didn't want to do that if it didn't work.
On Saturday morning I put the vinegar into a garden sprayer and sprayed the weeds in my front rocks at 8:30am. We left not long after that to go school clothes shopping. We got back at 1:30 and all the weeds that I had sprayed (and a small piece of grass that I accidentally sprayed were dead! So I consider it a huge success!! I took pictures, but I don't have them with me. But, even without seeing it in person or seeing the pictures I've convinced DH that there is absolutely no need to use Round-up or any other chemical on the rocks! This is huge because this has been a small point of contention for us.
So all the studies that said that 20% vinegar worked as well as Round-Up lied, it worked BETTER than Round-Up because Round-Up takes like 24 hours to kill the weeds and the 20% vinegar only took less than 5.
Now to find a place in town to get it... Or convince a place near me to carry it.
And just to clarify 20% vinegar is not the same as kitchen white vinegar. That vinegar is only 5-6%. Technically you can boil it down or freeze it to make 20%. I only specify that because a lot of people I've told asked me why I didn't just dilute kitchen vinegar to make 20% vinegar. The studies that showed 20% was as effective as Round-Up also showed that kitchen vinegar alone has a neglible effect.