Prof. Jeff Gillman has it right: using vinegar as a weed killer, plant disease control, fertilizer or soil acidifier is basically useless. And, potentially dangerous. To summarize the information in his posting at The Garden Professors:
• Vinegar only kills the top of the weed, not the root.
• That 20% acetic acid vinegar sold as a weed killer? Very caustic...to you, not your weeds.
• Vinegar not only does not thwart diseases, it could kill your plant!
• Vinegar as a soil acidifier or fertilizer is useless.
But it is tasty on a salad!