The said information: all those Bangkok movie theaters named after cities, states and countries, at one point or another came under the ownership my favorite film exhibition company, Apex. Originally, many of these theaters were built independently of the Bangkok movie theater heavyweight, but after being acquired by Apex the theaters' names were redubbed with an ear for geography. The Pratitpat Theater became the New Orleans, the Wong Wiang Yai Rama became the Hawaii, the Chalermpan Theater became the Chicago and the Klongton Rama became the California.
Following a similar pattern, when the Siam Theater (not the one on Siam Square) was purchased by Apex, its name was changed to the Las Vegas - today's showcase movie theater.
In addition to solving the mystery of the "location" theaters, Peep let us in on a bit of Las Vegas Theater history, as well. It opened in 1975 under the name the Siam Theater with a seating capacity of 750. In 1980 Apex purchased it and renamed it in honor of sin city USA. Serving as a second-run cinema for the duration of its operating years, which lasted into the late 1990's, the Las Vegas Theater stood on Soi Jaransanitwong 66/1 until just a few weeks ago when it was torn asunder. Nothing remains of this 1970's classic now except for a few photos and memories. Ashes to ashes...
If you enlarge the above Bangkok Post movie section from 1980 you can see all of Apex's theaters at the time listed under "Pyramid Entertainment," the Las Vegas included.