need a town midway between Vienna and Prague for a good lunch
There are many excellent restaurants in Brno. But Brno is a bigger town, it's not just a few houses next to the highway. I would recommend it if you wanted to make a real stop for a couple of hours, stretch your legs and walk around a bit.
There is a very good restaurant called Hansen in the center of the town. The address is Komenského náměstí 8. They serve traditional Moravian (= Austrian-Hungarian) cuisine, all fresh and natural.
If you didn't want to go too far from the highway, there is a restaurant quite close to the main Brno perimeter highway. It's called Valorie and they are serving sort of international cuisine. The address is Bohunicka 2.
Both should be open from 11am to 10 or 11pm without any break. They are both around 7-12 EUR per main course, Valoria is a little more expensive of the two. Both serve excellent food.