I have upset my friends.

Maybe she didn't want people to know. If she can't take a joke then that says more about her than you.

I once had people having a go at me because I asked this girl if she was in the choir, and she ran off in a flood of tears. Turns out she was being bullied about it and she thought I was making fun of her - in actual fact I wanted to join but wanted to know if I would know anyone first.

Having said that, the girl in question was and still is a silly cow.

buy a vape and vape in front of her, pretend you were looking for a vaping friend or smth and she took it the wrong way

buying a vape? kids have changed these days, tell her to get some **** the big wuss

Never knew vaping had a age limit.......i used to do it at 14-15ish.
But tbh not a big deal, but you kinda lost that friends trust and the girl who was going to buy a vape, is probably really embarrassed. You should try to be a little more sensitive around her it seems
Give her some time to blow of some steam (get the pun ^ ^?) and try talking to her that you didnt understand and that you had relatively good intentions and that it was supposed to be a harmless joke.

I recently started hanging out with a friend group that consists of about 4 or 5 people including me. I'm not that close with them.

So what has happened is one of them was talking to their other friend about buying something. Me and the other girls were waiting for her to come back and one of them whispered to me "she's buying a vape". I didn't believe her and she said she was just kidding. ( we are all under 16 ).
When she returned to us I jokingly and stupidly said "I heard you're buying a vape".

she instantly turned red and turned to the girl who said that to me and gave her the blackest look and barged past me and ran off.
The girl so said to me "that's so sly" while my other friends just stood there. i literally just said im going to my lesson and walked off and the girl who told me whispered to the others "so sly"I was shocked! like wtf just happened?? idk what to do and I feel really bad that I have upset them even though it was kinda her fault for telling me because I do have a tendency to say things without thinking.
Thanks and sorry it's so long