Michael said Carlo would be his right-hand man in Vegas. Did that ever happen?
“You have to answer for Santino,” Michael said.
Carlo is executed by the Corleone Family, specifically Clemenza, before the move to Las Vegas on Michael's orders.
The car pulled away, moving swiftly toward the gate. Carlo started to turn his head to see if he knew the man sitting behind him. At that moment, Clemenza, as cunningly and daintily as a little girl slipping a ribbon over the head of a kitten, threw his garrote around Carlo Rizzi’s neck. The smooth rope cut into the skin with Clemenza’s powerful yanking throttle, Carlo Rizzi’s body went leaping into the air like a fish on a line, but Clemenza held him fast, tightening the garrote until the body went slack.
The Godfather - Mario Puzo