Elizebeth poisoned charlie
Vanessa hands charlie the drink and she replies "tastes like gasoline" and vanessa chuckles and grins. If charlie is here all the time why would she react to the soda being bad this way if she had drunken it before.
She continues backstage and starts getting dizzy and drugged up so much she gets dizzy before aftons even near her, and is about to pass out and doesnt even react to being stabbed at the door other than her loud scream at the end. theres no way she went full sweat and pass out mode from lifting one boy... who mind you was also passed out until he woke up on stage.
btw by elizabeth i mean vanessa, just watched the full movie for the first time and its not terrible. not the best
EDIT: for extra clarification i dont think this makes vanessa evil, and it doesnt mean she intentionally did things. i think its very possible william told her to do things. "MAKE sure, she drinks this" she has to listen to her father, she is instructed by william to keep people away from the stage and near any place that is easy to spot william, charlie is given the drink because shes apparently always sitting here in the middle watching. he needed charlie to be distracted. vanessa was good friends with her so who else