Wow, so sorry this happened to you. You should probably refund her money. I don't think you'll find peace within yourself until you do. I can totally sympathize with the bride, but being a decorator myself can easily imagine myself in your shoes too.
I think what I'd do is talk to her and explain the details of what went wrong, just so she understands that you didn't just slap something together. Give her some understanding of how much time and money you actually did spend on this cake, then give her a refund. This way she will hopefully move on without harboring any angry feelings towards you. She really needs to know that you worked your butt off for her and that the end result was the product of your inexperience with boxed mixes and egg substitutes and such.
It does suprise me too that she wanted boxed mixes and frosting since they're not vegan! Wonder if she knows this? They both have milk products in them. Too bad she couldn't just have left her original order alone, but next time you have a vegan bride you'll know first hand what does and doesn't work and can make the best choice for them.
Sorry if I'm rambling, I just feel for you! This is something that I could so see happening to me. I have my Lucy moments all the time (Lucille Ball) and that's the way I imagine this in my head. I think the worst part for you is that you really did do the best you could and the result were disasterous. It's hard to work and work at something and then have no one appreciate your efforts. Hang in there. Next time will be better!