Seriously? Why are you even asking? Employers are not democracies. They set whatever rules and regulations they want as long as they aren't illegal. If they said you can't do it....don't. I'm Sure you probably want a different answer, but I'm Sure you didn't smoke at work, and it's not just you that works there. As a manager in HR, If hr is talking to you about not doing it, two things have happened.
1. MULTIPLE people have complained about your vaping.
2. YOU WILL GET FIRED VERY SOON IF YOU DON'T STOP.
I understand that we think it is harmless, but that doesn't mean it doesn't bother other people. Just because a place doesn't have a "No vaping" sign doesn't mean we can fire it up. That kind of arrogance is what will get it banned everywhere.