Interesting, could you please elaborate what you mean with that? Which brokers? And how do they get a better fill than anyone submitting orders electronically?
Thank you. Then it would be IB's best effort VWAP, because with the 0.017 cents per share exchange fee plus normal commission for the guaranteed VWAP, on low priced stocks, the cost can easily amount to 50 basis points one way. I don't know what the slippage is on IB's best effort VWap compared to e.g. MOC - I guess I will find out myself. Until then I will use good old manual limit orders during morning hours.
Its better to assess the difference between a broker's best effort vwap and guaranteed vwap price (aka tracking error). You probably won't find statistics regarding differences between MOC and VWAP. Ignoring market impact (as you stated) the difference comes down to the market dynamics during a given day. If you are not interested in assessing intra day momentum then you are better off using MOC imho.
I will try to ask for IB's best effort VWAP versus guaranteed VWAP tracking error and post it here, if they provide it. I guess there should be no conflict of interest, because it is the exchange fee that makes the guaranteed VWAP expensive - not the IB commission per se.
I am not sure but IB may guarantee vwap as part of their facilitation desk operations hence the fee may vary by size and name. Worthwhile checking also