Does the vinegar, salt and dish soap weed killer recipe actually work?
No better than plain vinegar and salt on their own, which is the part the recipe never tells you. The dish soap in it is a surfactant, not an active ingredient: it helps the liquid spread across and stick to a waxy leaf instead of beading up and running off. It does not add any weed-killing power of its own, so strip the recipe back to what is actually doing the work and you are left with the same vinegar and salt, with the same limits and the same risks to your paving.
The usual version doing the rounds is roughly a gallon of vinegar, a cup of salt and a tablespoon or two of washing-up liquid. The soap earns its place fairly: it is a genuine wetting agent, and a treatment that actually stays on the leaf works better than one that drips off onto the ground. But that is the whole of its job. It does not make the vinegar or salt penetrate any deeper into the plant, and it certainly does not carry anything down to the root.
The vinegar figure is where a lot of these recipes get vague. Ordinary kitchen vinegar is about 5 percent acetic acid, which is too weak to do much beyond browning young, shallow-rooted growth. Some online versions of this recipe assume you are using horticultural vinegar at 20 to 30 percent acetic acid instead, without saying so clearly, and that is a genuinely different, corrosive product that can burn skin and cause serious eye injury if handled the way you would handle the bottle from your kitchen cupboard.
The salt is doing more harm than most people realise. It knocks the visible growth back, but it also sterilises the ground it washes into, and on a drive or patio that means the sub-base under your paving, where it can stop anything growing there for a long time and is genuinely hard to flush back out. Add that to vinegar and soap and you still have not built anything that reaches the root, you have just built a slightly better-behaved version of a treatment that was never going to finish the job.
The honest verdict: this recipe is a marginally improved way to apply vinegar and salt, not a new or stronger weedkiller, and it inherits every limit and risk of both ingredients. For an annual weed in a crack it is harmless enough to try. For anything established on a drive, path or patio, a proper systemic weedkiller is still the only thing on the shelf built to reach the root.