The reaction should fire on value change. Change the source type to Selection. In your multi parameter use a blank as the first option then, A, B, C,... When the user toggles the multi parameter the reaction will fire your vb script. This is an efficient way instead of a rule as the rule is evaluated on every update.
Win XP64
R20/21, 3DVIA Composer 2012, ST R20
Dell T7400 16GB Ram
Quadro FX 4800 - 1.5GB