Why did Caesar choose someone as blunt and untactful to succeed him instead of someone smart like vulpes inculta

I disagree that Lanius is untactful.

He is actually a very well spoken antagonist and a brilliant tactician. So brilliant that Caesar viewed him as a much better choice to take Hoover Dam than this previous legate, Joshua Graham.

His intelligence is also on display in the final confrontation with him. He does not speak like a idiot, rather a dedicated warlord.

Lanius is the clear choice to lead the Legion across the Colorado and conquer Vegas as well as the rest of the NCR.

It's only after he has conquered the land that Lanius will fail. He is incapable of developing the civilization that Caesar desires to build on the west. But up until that point, Lanius is actually a very good choice to lead a war time military.

@princevegeta2005 Good question. I can think of three reasons.

One: The legion is a warrior culture and warriors usually hate trickery and deceit. A more sophisticated society might appreciate covert operations as a more efficient way to win a conflict or even as a way to save lives. Tribal warriors don't think that way. They relish in battle because it's the only way a man can be a man and die a good death. The use of trickery is an admission of inferiority, only women and cowards try to trap and deceive the enemy rather than facing him in open battle.

Historical examples of this are abundant, from Scandinavia, to Japan. Warrior codes tend to be extremely strict and shun anything other than martial prowess. Listen to how the legion speaks. They never claim to have a more fair, more free, or more prosperous society than the NCR. It's all about how they are stronger and everyone else is morally inferior.

So, how many of those people might accept as their leader a man who isn't a good warrior or general but rather a good trickster? A man who got to his position by disobeying his superiors?

Two: There really aren't better options. Backwards warrior tribes produce war lords, not good leader. Vulpes might come across as intelligent but he isn't, not in the same way as Caesar is. He is astute, and devious, but he doesn't have Caesar's mind or knowledge.

Lanius is a general. You can make the argument that he is equally as intelligent just in a different way. And generals are usually better to lead a militarized society.

Three: Not everything people do makes perfect sense. I think the devs wanted to show that to a degree.

In the real Roman Empire, Marcus Aurelius, lauded as one of the most brilliant rulers to ever lived, chose Commodus as his successor, even though pretty much everyone could tell Commodus was a total idiot, and corrupt, and vicious to boot. So why choose him? No one knows. Even today historians find it puzzling.

It may be that Caesar is just making the wrong choice, but would it really be that surprising?

@gavingold holy shit man well said. Definitely right about that aversion to trickery stuff, even in game you get pieces of info or references that make it clear that while legionaries are happy to have succesful attacks against they NCR they are always talking about full frontal assaults and crossing the river, and one even says some stuff about skulking in the shadows. Even dead sea who recognizes that caesar wants a stale mate for tactical reasons will turn a blind eye and allow you to go kill everyone in forlorn hope and be happy about it. Appreciate the thought out response

Also, Caesar knows well roman history, and warlords were popular leaders during the final years of the Republic (and caused a ton of blood to be spilled). Even if it isn't the most obvious choice to us, Caesar had his own reasons

Lanius is feared and well known, he isn't the brain but the image of what the Legion represent. Conquering, Brutalist, and merciless. Lanius will most likely have advisors like Inculta so he doesn't just go head first into every situation. Also I don't think Lanius isn't smart, there is strategy to even single hand combat that requires thinking, his size does indeed give him an advantage but that also doesn't negate the fact he's been through many ruthless battles with just a bumper sword. That man knows how to survive.

He didn't pick a replacement so much as Lanius was simply the next highest commander. He was the Legatus placed in charge of the Legions most crucial battle, he was the figurehead of the campaign. Leadership fell to him more out of circumstance than any proper planning.
Someone like Vulpes is smart, but isn't particularly well respected, or feared in the same way Lanius is. He was a symbol the Legion could rally around, much like Caesar. 
And for his part, he isn't an idiot. I often think people underestimate him. His battle plan against the NCR is going incredibly well. He's taking advantage of the glaring flaw in Oliver's plan, which involves trying to engage the Legion above the Dam, leaving the interior largely undefended. He is decimating the NCR flank, and barring the Courier's intervention, probably would have tore a hole right through the NCR's forces. Contrary to popular belief, he is willing to use unconventional methods to win, and is willing to adapt his strategy.

Vulpes may be more cunning but Lanius is the second most famous and respected commander to Caesar, if he was passed over or shunned by not being chosen as successor, the Legion would likely be less willing to unilaterally follow the new Caesar, or worse a civil war between Lanius and Vulpes supporters happens.