Limiting conditions, conditions, conditions, my friend. Otherwise nobody would be working on improving VWAP trading engines. You are basically asking for the holy grail. Its like me asking you how I should structure a vwap trading strategy: Should I buy the full size of the order if price drops intraday beyond 2 standard deviations or 3 or 4...10? If not full size, how much. In order to even start answering the question you need to introduce boundary conditions.