I was just passing through Vegas on a road trip with my brother, and he REALLY wanted to hit a buffet before we left. After some research into which ones were the most popular, I started calling around. Most of the hotel employees were not very helpful and told me (in a less-than-friendly manner) that I would be able to eat the vegetables (which I suspect are actually prepared with butter) and some items from the salad bar.
However, when I talked to the staff at the Bellagio and the Wynn, they both had me speak with their chefs who offered to walk me around and point out items I could eat. Both sounded very accomodating, but I ultimately decided to try the Wynn because their chef had indicated more options and an ability to specially cook items for me. When I arrived the food was delicious, there were several dishes to choose from, and the chef was more than happy to procure items like soy yogurt from one of the hotel's other restaurants if I desired. (He also mentioned that all of the Wynn's restaurants have at least one vegan option.)
The more research I did, the more I realized that Las Vegas is not very vegan-friendly, but here is at least one alternative that was satisfying for both myself and my non-veg brother. Hope this helps anyone else who ends up in Vegas!