A neurotransmitter called acetylcholine (or ACh) helps your muscles contract, as well as regulating functions as diverse as sweating, heart function, and pupil dilation. When everything is working correctly, ACh is a transient thing. It sends a message – for example, contract this muscle – and it is then broken down by an enzyme. It is important that, once it’s done its job, ACh get broken down so the muscle quits contracting. If it doesn’t get broken down, the muscle will be paralyzed.
VX throws a wrench into the body’s mechanism for breaking down ACh, wreaking havoc on the broad array of ACh-mediated functions found in the body. As such, it’s a potent poison.
The US has destroyed much of its stocks of VX. Today, chemical treatment and incineration are the preferred methods of destroying VX, but some of the earliest VX stock disposal was performed in the colorfully named Operation CHASE (Cut holes and sink ‘em), wherein ships were filled with unwanted weapons and sunk into the ocean.
Interestingly, some molecules much like VX have high enough toxicity to insects and low enough toxicity to humans that they’ve found use as insecticides. Malathion is one such example.