If you do not live in the apartment with them and it doesn't affect you personally or financially then it's best to stay out of it unless Amy asks you to get involved. It seems Amy is going to have to get a backbone and stand up to Vanessa or else she will continue to take advantage of her. If you cannot stand to sit back and watch her being taken advantage of and if you know it will not affect you and Amy's relationship negatively then I would talk to Vanessa and tell her she needs to pull her weight as far as bills and rent go but if you feel that Amy will get angry with you for talking to Vanessa then you will just have to be there for Amy when she needs you and if she asks your opinion then give it to her and be totally honest about your opinion of Vanessa and the way she treats her. You don't want Vanessa to come in between you and Amy's friendship she isn't worth that. It's really up to Amy to know when enough is enough and want to stop being used that isn't a friend, a friend will not use you to the point that you have to pay for everything.