Vinegar only tells you if the rock is calcium based, which could cause the pH to slowly rise over time if the regular tank pH is less than 7.8. Really doesn't mean a whole lot.
The main concern with putting rocks in a tank from a garden would be the use of pesticides, herbicides, fertilizers, and other chemicals that could potentially leech in to the tank. If you're confident that there haven't been any chemicals in contact with the water, then a good scrub is all you'll need to do. Never boil or bake rocks, as this can cause them to explode (if they have air pockets inside, which you can't know for sure).