From Optimal Trading Strategies :
There are two main reasons why traders execute orders using a VWAP trading strategy. First, a VWAP strategy is the trading strategy that minimizes market impact cost. Traders executing orders utilizing VWAP strategies are interested in participating with market volumes so to cause the least disruption to the supply-demand imbalance in each period. Second, for many traders their execution performance is measured against the VWAP benchmark. An average execution price more favorable than the VWAP price benchmark is considered high-quality performance while an average execution price less favorable than the VWAP benchmark is considered inadequate performance.
But, when it comes to backtesting, the market impact is thus assumed to be zero. One is assuming he does not affect the market as the VWAP benchmark price stays untouched. So how can one backtest his VWAP strategy effectively and more realistically?