VX is a chemical nerve agent, discovered in the 1950s by ICI chemist Ranajit Ghosh.
Like Gerhard Schrader, an earlier investigator of organophosphates, Ghosh found that they were quite effective pesticides. In 1954, ICI put one of them on the market under the trade name Amiton. It was subsequently withdrawn, as it was too toxic for safe use. The toxicity did not go unnoticed, and samples of it had been sent to the British Armed Forces research facility at Porton Down for evaluation and several members of this class of compounds became a new group of nerve agents, the V agents. The best-known of these is probably VX, with the Russian V-Agent coming a close second (Amiton is largely forgotten as VG). This class of compounds is also sometimes known as Tammelin’s esters, after Lars-Erik Tammelin of the Swedish National Defence Research Institute. The name is a contraction of the words “venomous agent X”.
VX can also be delivered in binary chemical weapons which mix in-flight to form the agent prior to release. Binary VX is referred to as VX2, and is created by mixing O-(2-diisopropylaminoethyl) O′-ethyl methylphosphonite (Agent QL) with elemental sulfur (Agent NE) or by mixing with sulfur compounds.
VX and similar nerve agents (Sarin etc) are liquid at room temperature and pressure, to make them airborne requires atomising the liquid. VX is a potent poison which is stable for a long time, so there was a military interest in using it to deny access to an area, however many countries (not DPRK / North Norea) have signed treaties agreeing not to use of the substance as a weapon.
News reports this morning were saying that the assassins were akin to suicide bombers in dealing with such a dangerous substance, but if exposure could be limited, for example forming the VX on the target, then the risk from the VX agent could be limited, and the main risk would be capture. In Malaysia being convicted of murder automatically results in the death penalty, which must be a risk for anyone suspected of being an assassin.
Source Wikipedia https://en.wikipedia.org/wiki/VX_%28nerve_agent%29
Source University of Birmingham http://www.birmingham.ac.uk/accessibility/transcripts/nerve-gas-in-warfare.aspx