Ya think ...

???
In fact, neither was Vito there at the homecoming dinner where Carlo expressed his dismay to Sonny & Tom at not having a bigger role in the Family. Remember also, it was Vito's order (on the day of the wedding) not to give him one.
It might be safe to guess that Carlo's discontent was well known within the Corleone Family.
Like I said...they DISCUSSED, PLANNED for months, probably more like years....prior to the moment when Carlo was named their man in Vegas.
Apple