Actually, in many places, employers/property owners aren't legally allowed to dictate what is in your personal vehicle, regardless of if it's on their property or not. Your vehicle is considered an extension of your home (and there's legal precedent on the subject).
Now... could your company fire you for vaping in your car on their property, and get away with it? Hell yeah: they just don't state the reason for your dismissal. That doesn't change the legal standing
I'm not familiar with Wisconsin law or the precedent you mentioned but I would venture to bet that may be the case in
some places, but in
most places, if an employer has a no firearms on premises policy and learns (and can prove) you have weapons in your vehicle, said policy would trump the "extension of your home" angle. I also believe that more often than not, if an employer fires an employee for violating a no smoking or no vaping on premises policy, they'll state exactly why they terminated the "rogue" employee.
Michigan is a right to work state meaning that, with the exception of the archaic union concept, an employer can terminate anyone for any reason at any time. And unless it's clearly an egregious violation of one's civil rights, the courts will usually side with the business
A company in Northern Michigan made national headlines several years back when they mandated that its employees would be terminated if they smoked anywhere,
even in the privacy of their own home. As ridiculous and Faustian as it is, the courts upheld the company's policy and people either found another job, quit smoking, or got fired.
For the most part, employers, like the government, can pretty much do anything they damn well please and make your life hell by dragging it through the courts long enough for you to lose interest & give up or go broke.
Sad, but true. God bless America, eh?