The S&P 500 publishes changes to the index in advance of the changes taking place, so all of the affected stocks have prices that reflect the change at the time of the change.
Most tracking indexes, especially the ones Vanguard uses, do not do that and thus do not allow the markets to front-run them. If you adopt the strategy of "do what Vanguard does" and you do it immediately after Vanguard says they did something, you are already too late to get the prices that Vanguard got and can kiss at least .05% goodbye just based on that. I would expect to under-perform by at least .25%, if not more.
Most tracking indexes, especially the ones Vanguard uses, do not do that and thus do not allow the markets to front-run them. If you adopt the strategy of "do what Vanguard does" and you do it immediately after Vanguard says they did something, you are already too late to get the prices that Vanguard got and can kiss at least .05% goodbye just based on that. I would expect to under-perform by at least .25%, if not more.