Rudi is right--check the fare rules for the tariff you are using.

For some, the clock starts with the first "international" sector, for which MEX-LAX-LAS would be fine--provided that the tariff does not define US-Canada and US-Mexico flights as 'domestic' for the purpose of that rule.

Remember, also, that MEX-LAX-LAS involves backtracking--LAX is west of MEX and LAS is East of LAX. Only certain fares will allow you to plug that into your ticket--even with LAX as a connection.