In Washington and Oregon -- and, I imagine, in the other states that have legalized marijuana, though I've not visited them -- you can go to the (pot) store and buy THC vape juice. Lab tested, certified, THC, THC-A, and CBD percentages listed, and source registered, with an audit trail clear back to the beginning available should there ever be a problem.
I think it is unlikely there is any butane left in the extraction, though -- butane boils at about the same temperature as water freezes, so there's no need to burn it off. Indeed, I remember a rather regular occurrence pre-legalization was house fires caused by the butane vapors from some amateur's extractor catching fire. Now, here, extraction is done in real grown-up labs by pros using real grown-up lab equipment. I haven't heard about one of those fires here in years. Still, I think you're on the right track; there are plenty of other possible solvents and pollutants that could have ended up in their vaporizer, whether from a bad source or even from themselves, and even from the legal MJ products. For instance, you can buy "tinctures" that consist of that extracted tar dissolved in alcohol. Fine to drink, if you don't mind your glass of wine tasting like somebody dumped their fresh cut lawn in it, but nobody knows what the precise chemical effects of putting that in a vaporizer would be. It's not an experiment I'd be willing to do! (Not 100% sure we have those tinctures in Washington, but they do in Oregon.)
While I'm on the topic, Washington State's research is showing big drops in teenager use of weed post-legalization. It's not a surprising outcome; being an illegal pot dealer in Washington is a recipe for loneliness and poverty now. The first thing you must do when you enter the pot store is show your ID and prove you're over 21. There's no "FFS, she looked 60," either -- everybody gets carded. It's pretty obvious, nobody wants to lose their shop because they sold to a kid. So much for the "Legalize it?!??! OMG! Think of the Children, They'll All be on The Dope!" crowd's pearl-clutching -- keeping The Dope illegal is actually harming kids, not helping them. (Gosh, that's the first time that ever happened with a GOP policy, amirite?)