God above there is some terrible advice on here.
I agree with one of the posters above (one of the minority of sensible voices) saying that this was going to escalate anyway.
Your daughter is completely within her rights to tell her established friends that vaping and smoking weed (these are 12 year olds!) makes her uncomfortable. This is not talking shit about someone. For it to be talking shit it needs to be either untrue or exaggerated.
Sit down with your daughter and go through the friendship group one by one. Who is on board with this, and who is also likely to be uncomfortable, but is just keeping quiet so they don’t get the same treatment?
I’m sorry the other mum was such a let-down. How well do you know the other mums? I can’t believe there is no-one else in the group whose mum would not be on board with stepping in over an older girl who smokes weed and bullies, joining the group.
For what it’s worth, I see no issue with you calling her a wrong’ un. Right now, she is wrong.
The fact that she’s doing things which are illegal means that you can make a safeguarding referral, for Cece and for the rest of the group. The designated safeguarding lead will be across this through the holidays, but will be checking more intensively this week so I’d crack on. Good luck - and sympathies from one ND kid’s mum to another.
Is your DD a member of any other social group? I’ve always followed Steve Biddulph’s Raising Girls tip that girls should always have more than one social group so if it goes pear-shaped, there is another setting without the stress. Guides?