From what you say, she definitely sounds like she's not well. Many people, vegan or not, are reluctant to take medication, but do you think she'd be open to therapy? Cognitive behavioral therapy can be very helpful and there is no medicine required.

I wonder if there could be something medically wrong as well. Vitamin deficiencies and medical conditions can cause or contribute to behavior problems. I was very deficient in vitamin D for a while (I live in Oregon. We get very little sunshine) and had bad depression and mood swings. My mood has improved dramatically since supplementing. It may be good for her to have blood work done to make sure her hormones and vitamin levels are healthy.

You need to look out for yourself as well. If she's making you unhappy and is unwilling to change or seek help, I think you may want to consider leaving the relationship. Best wishes to you. I'm sorry you're in such a difficult place.