We're adults and can make decisions for ourselves. If she wants to vape, that's her decision to make. If she has had doubts about being able to stay off of cigarettes (the greater evil) on her own, then vaping (the lesser evil) is a healthier alternative to smoking. The greater good of doing neither is also an option.
From another perspective, my 20 year old daughter never smoked, but all of her friends and brother did. When they all hung out together, she was the only non-smoker and admits that the peer pressure to start smoking had been strong.
She decided to vape zero-nicotine flavors as a sort of way to compensate for the peer pressures. Since she began to vape about a year ago, her brother and boyfriend have made the transition to vaping.