I'm Dutch and I can tell you guys will be fine.
As somebody mentioned their selection of players changes heavily every year, due to loans etc. Each year they lose their best players and need to rebuild their squad. (This season they lost their star players Traore, Pröpper, Vejinovic) I can't see Vitesse beat a way more solid Southampton anyway. Besides that, Koeman knows them very well, will be a big advantage.