In almost all MA training, there is a progression from technique to tactics to strategy. Because we all talk, we assume we have technique down for VSD and tend to jump to strategy, then doubt its effect.
As I get time, I'd like to present some real basics, VSD kihon. Hopefully others will follow suit and we can all start hammering out an effective system from the ground up.
Here goes: More basic than words are the rate, tone, pitch and volume of the exchange. We know if the threat starts talking faster, louder and his voice goes squeeky, there will be trouble. It's an effect of fear, of adrenaline of excitement. That's a perceptual skill.
As an active skill, if you deliberately lower your pitch, slow down and speak quietly, the threat will tend to follow suit.
That sounds really thin but try it! Just like any drill, you must do it. In your next casual conversation, start talking quickly and raise your voice... then watch the defensive mannerisms in your partner as he or she glances around to see who is watching.
The next time someone appears distraught, angry or just ornery, lower your voice, take a beat before saying anything at all, and watch them decompress to meet you half way.
Try it.
Rory