As I believe you already suspect, using the tar and resin left over from a burnt cigarette has some risks involved. The heavy carcinogen associated with smoking tobacco "Polycyclic Aromatic Hydrocarbons" (PAH), and several toxic compounds including "Acrolein" are actually created/formed by pyrolysis (when the tobacco is burnt) and will be present in the tar and resin byproducts of such incomplete combustion. They aren't actually found in tobacco leaf but are produced/formed when burning -ANY- organic matter, including tobacco.
Now that the politically correct obligatory disclaimer is out of the way, I would go on to speculate that the amount of those "nasties" you're going to consume when you vape NET flavored with your tar/resin extract is likely going to be pretty small. Perhaps the equivalent of smoking a single cigarette every month or two or eating flame kissed (grilled) steaks a couple of time a month. The funny thing about toxicity and harm from exposure is that it's always dose/time dependent. Rather or not the risk associated with using the tar/resin extract is worth the flavor is yields is completely up to the individual doing so. That's my ten cents worth.
EDIT: As far as trying to replicate the taste by flame scorching a NET, I don't think it would work (wouldn't impart a smoky flavor) but have never actually tried. Sounds like an interesting experiment!