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Following some advice from the web, I boiled vinegar and salt and applied it to some dandelions in our lawn. I probably should have used less of it though - I poured 2-3 tablespoons on the dandelions. Now the grass has yellowed around each dandelion - about a 3-inch radius.
Will the grass come back in the future? Have I permanently ruined these spots? What's the best way to fix them up?
Vinegar acts as a desiccant. It dissolves the protective coating on the leaves of the plant causing it to dry out and die if the temperatures are warm enough. It usually doesn't destroy the roots and sometimes the plant may come back if there is enough energy in the root system. Dandelions have a large taproot which makes them difficult to kill with something like vinegar. The effect of vinegar will not persist.
Salt is also a desiccant. It either binds moisture up in the soil, preventing the plant from getting it or it draws the moisture out of the roots when in the soil. Without water the plant will die. Salt will persist in the soil for a long time.
These are non-selective methods that will kill any plants that they are applied to. That includes not just the weeds but also any non-target plants, such as your grass, that come in contact with the application.
A better solution to remove dandelions from lawns is to use one of the weed pullers that are designed for weeds with long taproots such as dandelions. They're effective at pulling out the entire weed including the taproot and are easy to use. See my review of the Weed Hound Elite for an example.
As to restoring the dead grass.... I would dig out the affected area to try and get as much of the salted soil as possible removed down to a depth of 6" and fill in the hole with a mix of top soil and good compost (about 1:1 mix) and water it thoroughly to try and flush whatever salt may be left down below the root zone. It takes about 6" of water to reduce the salt content by 50%, 12" for 80% and 24" for 90% reduction according to this Colorado State Extionsion paper on Managing Saline Soils. Finally, sprinkle with new grass seed and lightly mix it into the top. Water as instructed for the seed you are using.
How much salt did you use? What was the ratio of salt to vinegar in your anti-dandelion solution?
Grass in general likes neutral to alkaline soil, and salt is anathema to most plants in general (although not all) so these will need to be either 1) diluted until it is no longer an issue or 2) counteracted in some way before the grass will be able to grow back into those spots. Since you added not just vinegar, but salt, to your lawn soil, I'm going to suggest that trying to correct the acidity using baking soda or some such is not going to be a good idea since it will just increase the amount of sodium (salts) in the soil, which would be bad.
My best advice is to be very generous with the water for the next few weeks in the hope that you can dilute out the vinegar/salt mixture you poured on the dandelions. If you used a lot of salt in your vinegar solution, however, this may only work to spread the damage around. Eventually things will stabilize with enough time and dilution (how long and how much depends on how much salt you used) but in at least the short term you are probably going to be looking at some ugly yellow spots where nothing will grow.
One other option just occurred to me - if the spots are few and pretty small you might be able to take a spade and lift out a large divot of soil where each dead zone is and replace it with good non-damaged soil and see if that helps the grass regrow into that area faster. This may be a practical solution if you only have a few dead spots to deal with. Obviously, if you have several dozen or a hundred this is probably far too much work.
Edited to add: Looks like my post crossed with OrganicLawnDIY's post. I think they are absolutely spot on and have a lot more info on how to go about removing the contaminated soil and reseeding the dead zones.
The vinegar will persist in those spots as a lower pH soil. Vinegar is used as an herbicide (a much higher percentage than what we use in salads) and prevents anything from being able to grow...great for driveways for instance. All one has to do is add lime to get it back into the pH range to grow plants again.
I would dig out the dead grass and soil, replace with fresh soil, test your lawn for pH. If it is below 6.5, use lime to raise the entire lawn's pH to between 6.5 and 7.0. If you haven't aerated this year, now would be a good time. Leave the plugs on your lawn. Use a hand spreader and reseed with a grass mix that says 'zero weed seed' on the grass seed list.
Fertilize (if you haven't fertilized for 2-3 months) with Dr. Earth's Lawn Fertilizer. Follow the directions. Use your hand spreader, not your hand to apply fertilizer. This stuff is worth the money, takes longer to show and lasts longer than the regular fast response inorganic fertilizers.
Mow no lower than 3"! This will prevent weeds from being able to germinate as the sunlight can't reach the soil. Also if your seed mix is like the Pacific Northwest's the grass plants have large root systems, genetically. Without enough top growth (3" minimum), the plant is not able to make enough food to support that root system and will be stressed. Stressed grass can not compete with weeds successfully.
Salt will be washed away. Don't worry about this. While the seeds are germinating (about 2 weeks after you over-seed) DO keep the surface of the soil moist, all day, every day. After your lawn has filled back in, water deeply and do not water again until you can step on the lawn and the grass doesn't spring back. This is training your grass to get deep roots and become drought tolerant.
UPDATE: Just thought I'd mention treating just the spots will cause the 'spots' to persist longer. That is why my opinion includes treating the entire lawn to prevent the original problem.
Boiling water or boiled liquid will kill the top part of the plant. The vinegar changes the pH drastically and the salt I have no idea why they put that in your recipe. Where did you get that recipe? Did they mean for this to kill dandelions in a lawn? I'm interested to see their thought process, truly.
