Vegan grub in downtown Dallas
Nothing comes to mind downtown, but I'm not vegan so wouldn't know the hole in the wall places. There are some Asian carry out places in the tunnels that connect the buildings, nothing too special, but you might be able to find something decent.
There is an Indian place called Taj downtown across from El Centro, it looks open sometimes, others not. Otherwise I'd guess you're looking at side dishes, strictly downtown.
However, a very quick/couple of bucks cab ride would get you to kalachandji's, a completely unique, mostly vegetarian, but also vegan restaurant run out of a Krishna temple.
Another option would be to take the red line Dart train about 4 stops north out of downtown to the Park Lane stop. Across the street from the station (behind the 7-11) is a place called Bliss. Its a raw bar. I can't reccomend it personally but the vegetarian/vegans I know like it.
I'd think you could get to either place, eat and get back in about an hour or so.