Ever tried modding your transmission? Set the Max Speed to 480km/h and play with your final gear ratio until it reaches that speed.
I don't think the Sebasten Vettel version lets you do this so be aware.
EDIT: It depends on the track and I don't want you going too crazy with it but areodynamics can also help you get a better top speed.
MORE DOWNFORCE = LESS TOP SPEED
In order to have minimum downforce Set the Aerodinamics to 50/50 (or 50/60) I dont really remember.