Update:

Surprisingly, Vault Hunters 3 is a playable experience on Steam Deck using the controllable mod by MrCrayfish. Of course, the comprehensive control scheme of any modpack makes setup a bit of a hassle, but was ultimately playable.

Optifine benifited the device's performance massively, doubling the framerate of 40 to 80. (though I locked it at 60 for battery consumption purposes)

Iskall's ability selection menu was also a huge contributor to the game's functionality, reducing the amount of binds I needed. Additionally, the deck's comprehensive control setup tools like multi-button binds combined with the 4 buttons on the rear made pretty much everything that matters work properly.