VSA works by applying the brake(s) to whichever wheel(s) are spinning or slipping, as in when you are stuck and trying to get free, or when the car's yaw (look up 'yaw' if you don't know what it means) detector senses that the car is yawing, in which case it applies brakes to whichever wheel(s) need to brake in order to make the car stop the yaw. So you might want to turn it off when you DONT want your wheels to spin. For me, these times are:
1.) When I am trying to go up hill in deeper snow and at non-highway speeds, and I don't want to lose power by having the VSA system apply the brakes.
2.) When the streets are icy and I need to pull out quickly from a stop, such as when I am trying to pull out into a gap inoncoming traffic. This is important, because if you are trying to move QUICKLY and VSA is on, you might find yourself in harm's way when VSA takes over and delays your maneuver for you. Turning off VSA in this situation is more of an 'advanced' skill that should be practiced after the driver is familiar with VSA and how it affects the car's behaviour. When it is slick, I would recommend that you go out into a safe place and practice with and without the VSA on, and then you can decide for yourself when you might want to deactivate it.
3.) Having practiced in 4 different Hondas with the VSA on and off, and finding myself very familiar with it, and already being a comfortable and experienced driver on snowy and icy roads, I will turn off the VSA if I want to exercise my own complete control over the car's attitude and behaviour, or if I want to slip and slide a little and have a little fun while driving, or if I want to go out and practice my winter driving skills in a safe location. VSA is a great thing though, and it can reall save your butt if things suddenly go wrong.
I anticipate that the safety nannnies will have something to say about this, but this is my well-considered opinion, and has always worked for me.