Lannister Vanguard at the Battle of the Green Fork.

[Abdallah]

At the Battle of the Green Fork Tyrion believes that the vanguard he is in was meant to get him killed. He disdains the men in it was basically the sweepings of Lannisport. They are lead by the mountain and he also sees freeriders and sellswords. But if one looks at who is composed of the Vanguard we can see why it managed to hold. First we know Tyrion has 300 Tribesmen. These are out of 3K that were basically the bodyguards of the tribal leaders. They are skilled raiders. Most viewed the tribesmen as cowards that "melted away when the knights of the vale road by". However there is nothing brave about being chopped up. The Knights of the Vale are heavily armoured, while Tyrion notes that the tribesmen only had stolen weapons until Tyrion gave them weapons. Chances are each one of them had killed a man before the battle. But there are 2 other groups within the Vanguard. Tyrion mentions sellswords, and there is atleast one sellsword company with Tywin at that time, the Brave Companions. They might be the worst scum imaginable, but being a psychopath has it's uses in battle. They must of fought in a previous battle before they came as sellswords fight for gold. They already had this bad reputation meaning they were hired before. They all probably killed atleast one man each. Finally if the Mountain is leading the vanguard then he's going to have his own men with him. The Mountain's Men, as they are known, are viewed as vicious dogs that take up after Gregor. For instance Polliver was able to fight Sandor Clegane in a pretty even fight. They too have killed men before and they won a battle ambushing Beric at the Mummer's Ford. So when u look at it the vanguard is

Tribesmen

Mountain's men 

Brave companions

all of them might be undisciplined but all of them are battle harded and experienced as well. Where as they are fighting exhausted Northerners. The last war the North fought in was the Greyjoy rebellion. But that was against the Iron islands. One can't fit that many men into an island, and when u consider Robert had the Crownlands, Vale, Riverlands, Reach, and Stormlands as well my guess is each lord only brought their best men. Some might of fought in Robert's rebellion but not enough. Tyrion wasn't in as bad a position he though he was. Sure he was surrounded by psychopaths, but Psychopaths are born for medieval warfare. 

[Universal Sword Donor]

As I've seen someone (alittledragon - name is prolly wrong) point out before, Tywin offered him a few roles before he let him in the van, at least one of which was letting him lead a group against the foragers. I want to say he also taunted him into protecting the rear, but that's more murky in my mind. I doubt that Tywin cared overly if Tyrion was in danger but I don't think he was trying to get him killed (or out of the picture) until he was in KL. Then it seemed a lot more open and hostile.

[dariopatke]

4 hours ago, Abdallah said:

 For instance Polliver was able to fight Sandor Clegane in a pretty even fight.

all of them might be undisciplined 

 Tyrion wasn't in as bad a position he though he was. Sure he was surrounded by psychopaths, but Psychopaths are born for medieval warfare. 

Sandor was drunk, but still impressive, remember what Greatjon did to Freys and he was way more drunk.

Undisciplined? I think Mountains Men are among most disciplined soldiers (in battle).

Well said, a man is much safer when guarded by Mountain than Ned or someone like him.

[Shouldve Taken The Black]

24 minutes ago, dariopatke said:

Sandor was drunk, but still impressive

Also, the Tickler helped, although Poliver did hold the advantage for a bit on his own if I recall, so yeah still impressive. 

[Trigger Warning]

It has more to do with Tyrion's Tyrion-centric world view. He wasn't even made to join the vanguard and was offered different roles first. 

[Shouldve Taken The Black]

5 hours ago, Abdallah said:

Tribesmen

Mountain's men 

Brave companions

To be fair though, as to Tywin's intentions, I don't think there's any reason to disbelieve Tywin's own explanation of his plan, which matched Tyrion's impression, that he expected the left to collapse. 

His main criteria for who should be placed there was that they were “undisciplined”. Knowing Tywin, he probably had a low opinion of the mountain clans, and he and Tyrion already had a conversation where it was clear Tywin thought they were undisciplined. Frankly, we don’t know whether the Brave Companions were on the left, but we know full well they’re undisciplined maniacs. Good in a brawl, or for terrorising peasants, sure, but undisciplined none the less. You could probably make a similar argument for Gregor’s men, but more importantly, Gregor himself is undisciplined, and he was put in charge of the whole division. Tyrion refers to him in that POV as a “fool who led with his rage” or something like that I think. The idea was that, however good the individual fighters may be in the division, they would lack discipline, and therefore cohesion, and get smashed.