Used as an execution benchmark: an algorithm that beats VWAP executed better than the day's typical participant.
The critique. VWAP measures execution against the market's own activity, not against the decision price. An order that took all day to fill can beat VWAP while the price moved away throughout - a good execution score on a bad outcome.
Implementation shortfall measures against the price when the decision was made, which is what actually matters to the portfolio.