You have forgotten about Saturn and Stussy. Saturn is the leader and Stussy can face a VA.
It's not underestimating the strength of vice admirals. Fans underestimate Zoro's strength. Some fans thought he was weaker than the SN captains and he wasn't. They thought he would oneshot by the yonkos and he wasn't.
Zoro obtained ACoC, just like Luffy, yet we are told it barley made him stronger even though he mid diff King with that power without going all out. Zoro is much stronger than some wish to admit.
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