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.
