Firstly, during the brief on the VX nerve agent, the formula on the screen isn't VX. Secondly, Malcolm says "Look at that chlorine bonding...": there is no chlorine in the formula on the screen. Thirdly, VX is C11H26NO2PS. No chlorine there either.
The danger of VX lies in direct exposure to the chemical agent persisting where it was dispersed, and not through its evaporating and being distributed as a vapour; it is not considered a vapour hazard due to its relative non-volatility. Even a bomb detonating barrels of VX, while lethal to those within contact radius, would not produce the cloud of gas stated to be the danger.
When Tom is crossing the bridge near the beginning of the episode he is walking from south to north, but he leaves the bridge onto the south bank.