One thing I'm curious about though is that:
Why is Legion so into taking the dam head on? Caesar answer's that. It'd be his Rubicon. Sure they've slipped into the area like mice finding their way into a home for winter, but if they come marching in like an army of ants to a new colony (other in new colony in this case being NCR) then it will incite fear into the enemy. It's more symbolic than practical.
They've taken the southern part; cottonwood cove, nelson, searchlight, nipton.
Taken is the wrong word. It's more about guerilla tactics, inciting fear to drop troop morale. 'Cold Cold Heart' is basically an example of what's happened in the soutern areas.
They could easily take the northern part.
Not so, the NCR would be able to defend the dam still, causing more problems, it'd be better to gain control over the dam first.
They have Khans, Omerta and possibly White Glove from the west.
The Three Families are all indicated to have been regional powers, likely residing in parts of Vegas before House emerged. As to the Khans, we know their roots.
So why take the dam head on?
They have the superior combat training and numbers.
Why not just circulate the dam, and trap the NCR in a corner?
Do you mean encircle? If so, then like I said, the Dam is easily defensible, plus it's the key to who holds the region.
They could surround them on all sides and simply wait for the ones in the dam to starve to death unless they try to make a break for it.
To many unknown variables. NCR could bomb them, send vast reinforcements, and on their side, the Legion would waste more food than neccessary feeding it's troops while waiting.
I just don't see why Legion has to go full frontal assault on the dam except for maybe honor and the fact that the final quest had to take place at the same place as the other paths.
The Legion, whether you will admit or not, were and still are nothing more than tribals, and tribes in history, usually found honor and glorious charges into battle as these things of bragging rights and increasing their reputation in the world. Like I said as well, The Legion has the numbers amassed, if they wanted to they'd easily just throw lives at the wall. With Lanius at the rear, they are afraid to just idly retreat.