Dian, 20% vinegar, if you could find it, is quite caustic and there is some danger to the applicator involved.

As far as I know, boiling vinegar does not increase the acidity - at least the ph doesn't change when using it in pressure canned recipes and canning where acidity can be paramount :)

If you use regular 5% vinegar as a weed spray it will kill minor weeds, especially if sprayed on a sunny afternoon (as opposed to a shady morning or evening). It will quickly kill top growth, and may not have a lasting effect on tap rooted weeds like dandelions. Unlike Roundup, it doesn't have a systemic action, meaning it is not taken down into the roots to kill at that level.