If Vash can heal from damn near any injury, why does his torso look all chewed up like he's been fed through a wood chipper?
Damn near any. He may very well have gotten eviscerated a few times.
In the manga, it turns out that while Vash can heal from his wounds, it uses his life force. Whenever a plant uses their energy, their hair slowly turns black, and whenever it becomes completely black, they die. Thus, Vash doesn't heal because he'd rather live with his scars than outright die.
Except that he never knew about the black hair thing until decades after he had the scars. It's likely he didn't even know he could heal, or if he did, simply chose to keep the scars as a reminder of the price of not taking a life.
If seeing Monev kill people drove Vash into Berserk mode, then why didn't seeing that Brilliant Dynamite Neon had killed the one train operator do the same?
Might be because Brilliant Dynamite Neon was acting like a terrorist with a cause. He had a reason to kill the train operator as messed up as it might be. Monev just killed a bunch of people for no goddamn reason which is something Vash literally can't comprehend.
In "Badlands Rumble", Cain is trying to lay low because Gasback wants to murder him, right? So why the hell does he think it's a good idea to create a 700 foot bronze statue with his goddamn face on it? It doesn't exactly scream subtlety.
I think when he first built it he didn't know Gasback was still alive or had given up the chase. Gasback had not attacked any of his former robbers yet. By the time he got it built it was too late. Either that or he figured there was no way Gasback could take out all of the bounty hunters.