Just my opinion- but, an OB would prefer that a woman who can't stop smoking be on nicotine replacement therapy instead of smoking (i was told while i was preggo). I personally, would take the risks of vaping over smoking.
It probably would be very hard to go straight to 0mg. Personally, I would wean off the mg. I would start at whatever she's at now- you said 10-15 a day? So assuming the cigs were .8mg each- that would be only 8-12 mg that she is smoking now. So I'd try to start with some 4-6 mg liquid, and see if that is ok. If it is, then I'd work on dropping it down til you get to 0.
And for goodness sakes, STOP smoking around her! (BTDT!) (since you're vaping, the point still stands, please don't do it, it's seriously mean to do to a woman who is forced into quitting)
Edited to add: I have 3 kids, and smoked during all the pregnancies. I tried to quit, and just couldn't do it. My kids were 7# 8oz, 9 # 8oz, and 8 # 6oz- all at term, or late, with normal placentas. But- I am just a fluke I think. My SIL had babies that were less than 5 pounds. Totally not worth the risk. I hated myself for smoking, and felt like a crack addict or something for being unable to quit, and was mad at my husband for smoking around me, and sabotaging my attempts to quit.