The US and the UN and basically everyone is watching his chemical weapons like a hawk and are just waiting to jump on top of them the minute it becomes necessary.
So that's at least hopefully not going to be a problem.
Sure the US would respond, the UN doesn't have any planes so they would ineffectually whine about it like normal. The problem is that once tge situation reaches the point where there is no down side to using VX then why not use it? If your back is against the wall you might as well gas the fuck out of the rebels and hope you can get tge upper hand.
Who gives a fuck if your going to hang tomorrow if you have a chance at surviving today. I am not saying that fucker shouldn't hang but giving him no way out just isn't a good idea.
The real mess will be once the regime falls what happens to all the chems then.