venting is where you fill your tank so it is totally full. basically your tank has a space in it to let the fuel expand if it gets warm. On the
ibiza's there is a button you can press when filling it which lets all the air escape so you can fill it to the top of the filler neck.
so a tank that seat says is 45 gallons will take something like 50 gallons. I have a leon myself and the only way i can do it is to wait till the pump clicks off then just fill it slowly untill the fuel is in the top of the neck, the level keeps dropping then as the air escapes so i just keep topping it up. it helps also if you can find a petrol station where its on a bit of a slope and park so your filler neck is the highest part of the tank. There maybe an easier way but nobody told me when i asked. If your going to have a go then i'd fill up when its quiet so you don't annoy everyone taking ages to fill you car.