No disrespect to foxyhorn, but I'm a vegetarian and don't eat at the Hobbit Hole. Ibiza and Farrago are in Midtown and are good places that have vegetarian items on the menu. There is also Cava Bistro and Mia Bella on Main, which are good, casual, and have vegetarian things. You may want to check what time they close though. Ibiza and Farrago probably stay open later.
I recall that Bossa is not bad (click the link for Houston Bossa). They serve dinner until midnight on weekends; bar is open later--maybe a limited menu there?
I had a decent chimichurri steak there. I wasn't really paying attention to how people were dressed but I think I was just wearing a skirt and tshirt, so it's pretty casual iirc.
For late night Vietnamese, there is Mai's on Milam in Midtown. Another place worth mentioning is Dharma Cafe in the Warehouse District by Last Concert Cafe, which could also be an option.