Houston and Las Vegas are about 1,226 miles apart by road. Split the difference and you arrive near El Paso, the city closest to the halfway point between them. That puts roughly 14 hr 8 min of driving on the Houston side and 12 hr 12 min on the Las Vegas side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. El Paso still makes a fair, central place for Houston and Las Vegas to converge, splitting the travel instead of asking one side to cross the country.
If El Paso doesn't have what you're after, Albuquerque and Tucson are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Houston and Las Vegas.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.