If the offence ONLY include smoking/vaping on the plane -- however it sounds like they also did not comply with the air crews admonishment and refused to comply with instructions. As such, I believe that the US federal law of "Interfering with a flight crews duties" which includes everything from physical interference, verbal interference, and refusing to comply with instructions (including fastening seat belts, stowing luggage, turning off electronic devices, and observing no-smoking rules) or other disruptive behavior... (49 U.S.C. § 46504). For these the civil penalties are up to $37,000 per violation (multiple incidents can come from a single incident), and also criminal penalties (up to 20 years). This is of course a lay opinion from the plain reading of the law.