Maybe 10 train cars of formaldehyde is fine, maybe not, but, this is something that should be assessed and not dismissed out of hand, even during discourse. Random internet commentators probably don't really know if it's fine. And despite the people on-site saying measurements are fine... animals are getting sick, which is the classic warning sign with chemical spills that things are not fine.
Although there is indeed dioxin formed during the combustion apparently. Bear in mind too that this isn't a controlled incinerator, there could be various things produced at different temps etc.
When left to break down in the environment, after several days vinyl chloride will break down into a combination of formaldehyde and hydrochloric acid. The latter of which won't be harmful, but the former will be.
It will also be destroyed in the soil by any number of mechanisms, with a half-life of about 14 days. [2]
[1] https://www.atsdr.cdc.gov/toxfaqs/tfacts111.pdf
[2] https://webwiser.nlm.nih.gov/substance?substanceId=1&identif...
Erm, on what basis do you make that claim?
Ten train cars of dioxin spread over a square mile is a lot. Spread over 100 square miles? Probably still a lot, like the place in missouri that dumped it on all their roads to keep the dust down. That's probably that same order of "10 cars over 10x10 square miles", of a sufficiently nasty thing.
10 cars = safe is a number that needs context and validation and it will depend on the particular chemical.