Plain vinegar will kill any plant without adding anything else. But thistle is a very tough plant with deep roots that spread underground, and the vinegar will not kill the roots. So it will come back. You can keep killing it, making sure to get every bit of green that emerges, until it gives up the ghost. Without photosynthesis, it will have no way to keep growing.

But the best way to get rid of it is probably to deeply soak the area with water, and using very tough garden gloves, try pulling it out along with as much as the root system as you can get. Thistle will grow back from bits of root left in the ground, so you have to keep on top of it.

For some reason, people love the idea of mixing concoctions of various common things like soap, Epsom salts, salt, oil and vinegar, but it often just makes a mess or hurts the plants you're trying to save. You're right. If it sounds too good to be true, it probably is.

I tend to be wary of anything that has "miracle" in the name. That said, I use plain vinegar as a foliar spray to kill weeds, and it works as well as roundup with one benefit being it's a heck of a lot cheaper. As far as thistle, I got better results with pulling them out after a good rain, but if your soil dries out as quickly as mine does in summer, spray some vinegar on the leaves (for more dramatic results, microwave the vinegar so it's hot, but no so hot as to melt the spray bottle). The vinegar will kill the surface plant, and it you're diligent about it you will eventually get less thistle coming up, though nothing will stop the seeds from blowing in...

Maybe we have tougher weeds down here, but in my experience, even drenching both plant and soil with vinegar only kills the visible part of the plant, if that (and not always even that if you use vinegar from the grocery store). It's useless for serious weed control.

You have good instincts, Steve.

Thanks everyone for confirming what I thought!

Steve