Brokers guarantee VWAP only if they also are paid a very healthy commission. They generally lose to VWAP, but makeup for it in premiums.
Plus, if you are dealing with a large broker they may just cross your order with an offsetting order (or their own position, execute nothing in the market and keep the entire premium.
Plus, if you are dealing with a large broker they may just cross your order with an offsetting order (or their own position, execute nothing in the market and keep the entire premium.