We told Cunard in advance that we are vegans and they were helpful and enthusiastic and sent us a written confirmation of our request. When we arrived at dinner in Britannia the first night, the crew was completely surprised and seemed to scramble for a moment. That said, they took great care of us and were able to veganize all of our meals. I mention the disconnect between the booking agents and the crew only to point out that, although they weren't prepared in advance, they were still able to accommodate us. I would definitely tell Cunard in advance if you're a vegan, but they can easily handle you, anyway. Oh, and they were able to find and serve us soy ice cream for dessert the first night (and every night if we wanted it)- they are ready!
A few notes:
BREAKFAST: In the Lido buffet, you can do a vegan full English! The chefs confirmed that the beans, mushrooms, grilled tomatoes, and hash browns are cooked in oil, not butter. They have plant-based margarine out as a matter of course, so no need to ask. There is soy and rice milk for your coffee and cereal, both in the dairy-free section and in the carafes near the coffee. Halfway through our trip, in Venice, we noticed they offered soy yogurt, as well. This wasn't picked up for us- they have a small gluten-free and dairy-free section in the Lido- both of which seemed popular with guests. Note the dairy-free section isn't necessarily vegan (some things were just lactose-free) so ask if you're not sure. The soy yogurt was clearly marked vegan. Vegetarian sausage is on the room service menu and offered in Lido, but they ran out, unfortunately, do I don't know if those are vegan.
LUNCH: Most of our lunches were in port, but the Lido has many options for sea days. There are many items marked "vegetarian" but a few are also vegan-just ask. A number of the Indian curries were vegan (except things like korma, makhani, etc) and, along with the big salad/fruit bar, we ate well. They offered hummus one day, they had some bean salads on others. You can make it work. They had veggie burgers outside at the grill near the pool, but I didn't ask if they are vegan. Maybe someone else knows?
DINNER: We really love Cunard and dressing up and eating in the Britannia for dinner (we skipped it for brekkie/lunch as we felt this would be a burden on them and we'd have fewer options). I will be honest here: Cunard veganizes their meals the same way many restaurants do; they simply remove the meat/cheese, etc. This often leaves just a small, pretty plate of vegetables and a starch- and you hungry. The food was great (and the service was phenomenal), but we often snuck into the Lido after dinner to find another curry, bean dish, or something else with protein to round out our meal. If this bothers you and you're ok with the Lido, you'll find more options there. All this said, our waiters in Britannia could not have been more helpful or more attentive to our needs. In fact, the head waiter caught that we were served a small side dish of raita with our meal one night (we didn't mention it). We said it was fine, we didn't want to make a fuss. He said, "No, madam, it is NOT ok!" We didn't want to get anyone in trouble- they're busy and mistakes happen. My point is no one forgot our preferences in the two weeks (maybe except one busy chef once...) and they went out of their way to accommodate us. They couldn't have been kinder or more helpful. You will be well-cared for as a vegan.
When you request a special meal, the head waiter gives you the menu the night before so you can make your choice and prepare the chef. As a vegan, they give you the next night's menu as well as the standard vegetarian menu (same every night, but full of choices) and they indicated they could veganize anything on it. In most cases, they simply removed the non-vegan part of the meal. In a couple of cases, they slightly changed the meal. Again, it all worked out just fine and the meals were delicious and well-prepared.
OTHER NOTES: We did purchase some snacks in grocery stores on port days (nuts are a good protein supplement) for our cabin. Some stores, like one we found in Venice (Conad), had tofu, seitan, and vegan cheese. If you're really missing these items on your trip, you could grab those, too, for a cabin snack.
We found vegan restaurants (with vegan meats and cheese!) in many of our ports for lunch (Athens, Rome, Dubrovnik, Kotor). I won't detail those here, but feel free to message me. (as a side note, one of my American Airlines vegan meals had vegan cheese. It seems airlines (and maybe Cunard someday, too) are discovering the replacements slowly...).
CRUELTY-FREE: The toiletries are Penhaligons, which doesn't test on animals. None of their products on board have animal ingredients but, as a company, they do use badger hair in brushes (http://www.penhaligons.com/frequently-asked-questions). My pillow was synthetic; I believe the other bedding was, too. I obviously didn't tear anything apart to verify, but I'm sure other folks with allergies have already posted in other threads about this. I think Cunard offers down on request only? Maybe someone else can help with that.
FINAL ADVICE: Please tip well if you are a vegan! Although they were well-prepared for us and other special meals, they are going out of their way for you and it's extra work for them on an already long day. The staff on board is phenomenal and a great way to thank them for their extra efforts on your behalf is to tip them well.
Enjoy your cruise!