El Nuevo has vegan menu options throughout (marked "V") and they use Daiya cheese as well as local seitan heros, Upton's Naturals (chorizo seitan and chicken variety).
For appetizers, one could start with guacamole and chips. When I went, I asked them to make vegan quesadillas for an appetizer (as they had regular on the menu and I knew they had vegan cheese) and these were good although it's a lot of Daiya. I'd recommend splitting with a friend for sure!
The quinoa that came with the entrees was spiced like Mexican rice so it has a kick to it, be warned! I prefered the white rice instead. It is nice they offer another type of grain other than rice, though. I do like that the entree's generally come with a spring salad mixture of mango and jimica. It gives the meal some lightness to it, especially if one gets the fried entree's. The flautas were really good, although I am not a huge fan of chipotle! Another dish used Upton's Naturals chicken style seitan that was marinated and put into the mini tortillas with some vegetables. I really liked these! I just wish I could find it on their menu online. Perhaps it was a new item when I went there.
Pricing is a bit on the moderate side especially if one gets appetizers and drinks. I would prepare to pay at least $60 for a total bill of two people with appetizers, entree's, and drinks. If you want a nice Mexican dinner, though, I very much recommend El Nuevo Mexicano!
For a complete review, please see my blog here: http://veganmiss.blogspot.com/2012/06/vegan-chicago-restaurant-review-of-el.html