Wikipedia has something interesting about VX:
> 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) as is done in the Bigeye aerial chemical bomb.
So maybe two of them were holding different component and they only reacted after being sprayed on Kim's face. If this is true then this is a very intricate assassination plan.