Maybe you do understand, but let's break it down anyway.
Vegas mode is as follows: Three lanes, two shoulders, as is normal on most casual and pro modes. You need to head into the shoulder to activate Vegas's feature, the shuffler. There's nothing else you can do with the mode, outside of pick up blocks normally.
On Iron Mode, the shoulder is removed. So it's impossible to use the shuffler, and thus, it stops being Vegas, and turns into a three-lane no power at all game. If you wanna play it that way, I guess one could, but it's not VEGAS.
-Lala-