I visited La Zanhaoria for dinner last night and was impressed with all the vegan food they offer, but disappointed to arrive at 7pm, as most items (the large buffet) is only served until 6pm. The restaurant is in the Zocalo (center) of the city and business largely dies down after 6pm so a more limited menu is offered then.
I'm surprised at the previous comment stating that this restaurant also serves meat. I didn't see any meat being sold in the to-go fridges or packaged food area and the staff told me that all "meat" on the menu was vegetarian so I'm hoping that the author of the previous comment simply misunderstood.
The to-go deli up front doesn't offer many interesting options for vegans, unless your thing is a "ham" sandwich with no cheese and no mayo - basically, soy on a bun. Meh. The restaurant in back, however, is quite another thing. The daily special was vegetarian paella, which was only served until 6pm, along with the buffet - disappointment! However, I went with the gluten milanesa with pipian and was pleasantly surprised. The food came out room-temperature and the waitress didn't come back for 15 minutes so I couldn't ask it to be reheated (annoying!) but I really enjoyed the flavor of the pipian sauce, which I had never had before. It's a creamy (non-dairy) peanut sauce and was served over the milanesa (I believe it's a breaded "beef" chop), alongside "arroz integral" (brown rice) and "salad". Well, the brown rice was pretty over-cooked and the "salad" was just iceberg lettuce and tomato that weren't very fresh. I also had the tortilla soup which was good.
I would normally give a restaurant only a "fair" rating based on this experience, but given there are so few veg restaurants in Puebla, and in Mexico in general, I think it's worth visiting. You will definitely get more interesting items than you would get elsewhere.