I don't know about them being harmless, but they're way less harmful than cigarettes.

It's a perfect is the enemy of good kind of thing. Less people are smoking cigarettes, which is good. But they are using vapes so it isn't perfect. So let's make some laws to make things perfect!

Either the law isn't followed and people still vape, or it is "successful" and people go back to smoking cigarettes. People are going to do things that are bad for them. Better to have them do the lesser of two evils.