Is it dangerous to boil vinegar?

Nope, not at all.

Nope

We used to boil vinegar to color Easter eggs

Plus my wife cooks with vinegar all the time, her adobo is great

why the hell would it be??? most all foods that include vinegar(pickles, and other condiments) have been boiled...I used to can lots of home grown produce and boiled the vinegar based brine all the time...still here, turning 68 in a month....healthy...alive...lol.

Not at all. 

Obviously, you haven't cooked adobo yet.

No,  I used to boil vinegar to make mint sauce.

It can produce fumes that are pretty harsh and make it difficult to breathe.

In general when vinegar is boiled it is a mixture of water, sugar, salt and spices as part of a brine. So the fumes are not that bothersome. 

I would not pour a gallon of vinegar in a pot and boil it. It would shine up a stainless steel pot but may damage aluminum or cast iron if left in the pan for a long time. 

Only if you breathe in the fumes. 

Boiling vinegar produces fumes and if you are sensitive to them it could bother you.

The fumes from boiling vinegar can kill your pet bird, so if you have a pet bird, or anyone in your household has lung or breathing issues, I would avoid it.

I shop thrift a lot and sometimes used to use boiling vinegar to clean enameled cast iron, but I have a parrot now so am super careful about any sort of fumes.

Why would it be dangerous? It's not like it's gonna explode or something.