I have 2 fields that the user fills in, a data type field, which has dropdown choices to indicate text, numeric, or date, and a value field which user types in the actual value.
When the data type field indicates a date, I want to give the calendar VA to the user, but if the data type field is not a date I want to suppress the VA.
Another way might be to have a date field, a numeric field, and a character field overlapping each other on your form. When the user selects a data type, selectively show the object with the correct type and hide the others.
Go into the properties of your custom field, select overrides, data dictionary overrides, press the Visual Assist push button in the overrides section and select a visual assist search form (any will do). Once you've done this the visual assist event rule should start triggering.
I know this is late and you've probably already determined a solution. I once had a data item where I had to override a Visual Assist and then in the Visual Assist Button Clicked ER change the action, otherwise I couldn't get the ER to trigger. It seemed weird to attach a Search form that wasn't going to get used, but was necessary to trigger the ER.