1st: vwaps do not have to start on the open (you can place a vwap order at say 11 AM). 2nd: How would you know a VWAP order is taking place? How would you know it is a buy order a sell order or any combination thereof from multiple brokers. 3rd: If a stock drops below the VWAP and you use this as a signal to buy - GOOD luck. If the stock continues to sink they get the average price on the day. You just get a loss.
QCharts tracks real-time VWAP and those farthest above/below if you're interested. I agree with Def, I'm unable to see how VWAP is any sort of indicator for entry. I had always heard it was primarily used by mutual funds. I also read in a book a few years ago that some MM bonuses or incentive pay was tied to their performance against the VWAP.