If only one person takes up smoking again as a result of a vaping ban and later dies from smoking-related health issues, such as heart disease, lung cancer, or emphysema, then any organization or individual who was responsible for the vaping ban was complicit in that death. You seem to be arguing that if you only kill a few people, then you're not really a murderer, or if you only contribute fractionally to the murder, then you're not a murderer. Technically, the definition of manslaughter is taking action that results in the unintended death of a human being, as opposed to murder, which is the intentional taking of a life. So, the question is whether BT knows that their actions will directly or indirectly lead to one or more human deaths. If so, that would permit characterization of BT as murderers, and if not, then they would only be manslaughterers. You might also consider this as a conspiracy to commit murder, which doesn't require any direct act on the part of the conspirator. Alternatively, you might also consider the FDA to be guilty of negligent homicide, since their failure to promote vaping as a viable alternative to smoking lead to the death of a person without malice.