Except a huge flaw in that problem is that Vegas will change Caesar's Legion. And that having Legion tear itself apart will take hundreds of years, like the real Roman Empire. They'll die from hedonism, laziness, and the lack of a challenge like the real Roman Empire.
-How about no. Caesars legion depends on gobbling up tribes, submitting them, turning them into recruits to get more land. If he stops after vegas, he will alienate his own indoctrinated officer-corps, his capital will be within artillery range of the NCR, His nations capital will be on the very outskirts of his realm with long traderoutes ect. It would make zero sense for Caesars legion to stop. If it stops, it dies. Only way to starve off the inevitable demise is by continuing its modus operandi... keep gobbling up tribes.
The "Real" roman empire didnt rely solely on brute force. It relied every bit as much on bribery, debauchery, politics, great infrastructure, pursueing technological advances in medicin, sanitation, construction ect, a firm and elaborate bureaucracy, "citizenships of rome" ect. Caesars legion lacks all of the above.
Caesars legion is a perversion of the Rome and Spartan societies... he gets all of their vices, and practically none of their virtues. Smaer poo on a stick and poke a centurion and he dies from infection and gangrene. No matter how much of a toughguy he is.... because the legion doesnt believe in medicin... or anti-biotics. Good luck getting alot of sons when your women risk death with every birth, hell the birthrates of the BoS must be higher even if they are inbred after 200 years of only socializing amongst themselves.
If caesars legion is to have a chance it has to have some radical leaps of reform. It could be done by breaking the kingdom in two, with the west roman empire crumbling and the east roman empire (byzantine) reforming and making a new Legion. As it stands now, Caesars legion is a colossus on clay feet. Im surprised the MWBoS hasnt wiped them out.