I've yet to see or hear of a dog drop dead from the wiff of an onion, clove of garlic, or vinegar.
The chemicals in many popular flea and tick medications and products, however...well, they have quite a different track record. Many of the synthetic chemicals used actually work because they interfere with the insects' nervous system. Either by blocking chlorine and therefore causing paralysis or by simply blocking nerve transmission completely. And given how many neurological symptoms have been documented and warned against while using these products...hm. I'll personally steer clear, and that is a decision I'm quite happy with.
There is nothing inherently dangerous about onions, garlic, or vinegar. If there were, I would think we would certainly know and plenty of dogs would be dead by now, given how many food companies put onions or garlic in their formulas.