No I didn't forget Saturn, you are the one who seems to have forgotten he is not just a mere soldier if you expect him to go fight on the front line like the navymen. He is too important for that and doesn't want his presence to be known. He will get involved eventually yes, but only when there is no other option. Obviously Oda didn't bring such an important character all the way here just to fight, his role is gonna be more than just that, most likely related to Vegapunk in some way.
And stussy fighting a vice admiral ruins the imagery of the Yonko's 9 commanders taking down one vice admiral each. It's just too big of an opportunity to pass up, there is even 2 female VA to match our 2 Nami and Robin. It's obvious to me what's gonna happen. However strong Zoro is, Oda can match him against a VA, it doesn't need to be an extreme diff fight