Like witnessing a murder?
How about a listicle about murder on a website like Buzzfeed or EliteDaily?
Unlikely, but yes, anything is possible.
How much trauma is needed for a "healthy" person to become non-responsive?
It depends on a person's threshold of pain.
Can this happen through auditory/visual stimulation or does the brain need a physical trauma to become affected?
Anything that was anxiety-inducing.
Maybe a website that primarily publishes GIFs of horrific car crashes?
Again, unlikely, but possible.
How do you fix something like this? Medications? Therapy?
Yes, therapy and then medication if it's necessary.
So, theoretically, an insane fact nestled within an article rife with shocking GIFs could scare someone so badly that they fall into a vegetative state.
I mean, it would be unlikely, but anything is possible.