Vans come up a little bit bigger, as do a lot of surf-style shoes. If they don't have a 10.5 try the 10. If you go for the 11 and they do slip around, your heals will be in agony after a day! If the 10 is a little tight at first, bare in mind that they'll loosen up a little.
Actually, You should try getting a 10.5, But An 11 wouldn't do any harm so try to get a 10.5 first then get an 11 if they don't have any in stock.