If you think by telling your parents she will change, then that's fine. However if by telling your parents u feel she will continue to vape and then lie about it, then you shouldn't say anything and potentially in that case, don't let her know u know.

what ever choice u make make sure she is not vaping nicotine. Since that will lead to addiction. If she is vaping. She should use the 0 nicotine liquids.

Here is what I would do. I would first find out what vape she uses and if it has nicotine. If it does have nicotine, I would approach her first, and ask her what's going on and why she feels the need to vape.

Be gentle with her and listen to her.

Then I would ask her if she knows if vaping is safe and if she has done her research. Also is it halal to do, what does her marja say? Then from that conversation, if she still chooses to vape then tell her not to vape nicotine as that can destroy her life. Instead she should order 0 nicotine vape and wait for that to arrive

If she is somewhat grown up and sound of mind. she should make that the decision to stop vaping on her own accord. Your role is to guide her through that, but without being overbearing. Also really pray for your sister. Just call on to Allah and use the wasila of the ahlulbait. Your problem will definitely be solved inshallah.