How would one go about making flex/vertex animations controllable?
 

Similar to GMod's voicechat (in which a lua file calls a flex animation for voicechat), how can one replicate such concept in Sourcemod?

Re: How would one go about making flex/vertex animations controllable?
 

you would have to forget about flex and just recreate the mouth animation with sprites or models that you parent to the entity. Hook voice data stream and processes it so you know which sounds the audio stream is making to animate the mouth\sprite.