I'll specifically recommend against Garden Grill. We're vegetarian, not vegan, but even we couldn't have a great meal there. I'll also advise against the other tier 1 restaurant, Biergarten (despite it serving some incredibly excellent red cabbage). While both of these restaurants could make a unique effort to satisfy vegetarians, they haven't done so for us in the past, and I wouldn't count on them doing so this time. They're really all about the batch dining approach - cook a lot of a few items, and present them family style or buffet.
Tier 2 offers San Angel Inn. I've said a lot of negative things about San Angel Inn over the last twenty years, but it has been very steadily improving, so much so that I stopped talking about it about five years ago, and it has now made it onto my "recommended" list. They generally only have a couple of vegetarian options, but they're sufficiently representative, and good eats. However, I'm a big cheese fan, so I'm not sure my recent positive experience at San Angel Inn will translate as well to a vegan dining experience. I like spicy food mellowed by cheese - remove the cheese and you still have a nutritious, filling meal - but you no longer have that mellowing that I find makes the meal super.
I've been disappointed with Marrakesh over the years, but mostly because they water the food down so much that it isn't anything any self-respecting Morrocan would recognize as their own cuisine. Also, since I haven't eaten there within the past five years, I can't speak to how well it will serve a vegan's requirements.
I've been to Nine Dragons once, over twenty years ago, I believe, and it was so bad, and reviews of the place have been so consistently mediocre or worse, since then, that I never went back.
Rose and Crown may be able to offer a vegetarian shepherds pie that would meet a vegan's requirements, but I'm not sure you can count on that one item being available, without making arrangements in advance (which you can surely do, and if you do, I think you'll be in great shape).
Would you consider a tier 3 restaurant?
