We were on a lux cruise line that promised to honor any diet, but we quickly learned that vegetarian strictly mean "no meat". There was often butter or cream in the dish.
If you want vegan you have to very clear to ask for it.
I think most cruise lines now ask you to specify a diet in advance before your cruise - you should be able to do it in your cruise profile online.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.