If their interest in the area are threatened, they would defend it. If Helios One, the Hoover dam or the Mc Carran camp were about to be taken, they would defend it, of if it is taken, they would retake it.
But on the other hand, the common line for them to handle annexions is firstly political, not military.
- For the Vault 15, instead of attacking the khan and the squatters, they let prefer negociate with the squatters.
- For Vault City, they use a third party (Bishop and the raider) to create a situation in which Vault City has to consider their protection. They don't attack it themselves.
- For Redding, they hope that their most supportive candidate would win the election.
- For cleaning Jacobson from super-mutants, they use mercenaries to force super-mutants to make their move. (leave by themselves, or attack the mercenaries, providing a reason to invade them). Even the mercenaries don't attack.
- For Primm, they don't protect it, as long as Primm citizens don't ask for protection. If they ask for protection, they will be invaded and forced to pay tax. (even if they're responsible for that mess in the first place, by letting the prisoners escape)
- For the strip, they don't attack the securitrons or the gangs, but wait for an occasion to overthrow Robert House himself.
- They do intevene to retake the Prison, but that prison was their from the beginning, and the powerganger were their own mess. But even there, they don't move beyond that strategic point. They don't hunt the prisoners that left the prison.
So their non attacking behavior is consistent. If there is another option, they wont attack other cities/nations/people, unless there is a strategic point involved.
Beside that, they're not at home. They have to look at every corner, has if they make any mistakes, they would have not only to fight the legion, but also House, the Kings, the Great Khans, the fiends, the brotherhood of steel, and any other NCR haters. So they have to be carefull and cautious.
On the Other hand, the Legion is already at home, on the east side of the river, don't have to worry about public relations, as Caesar is a god, and have countless troops, so he can zerg rush the NCR, like the NCR did with the Brotherhood of Steel. While the NCR has to secure its positions in the Mojave, Caesar use his spies to gather intelligence about them, and only them.
Also, their attack agains't khans on bitter spring left them with a disastrous public image.