-dx12

-dx12 in the launch options. The game runs better in Direct X 12 than Direct X 11.

I have played the game using fairly constant 60 fps in handheld mode and when docked. Here are my settings for each one these options.

Handheld Mode (60 FPS)

Visual Effects- Medium

Display Mode- Windowed Fullscreen

Framerate- Unlocked

Vsync- Off

Anti-Aliasing - TAA+

Ansiotropic Filtering- 4x

Distance Scaling- Far

Texture Quality- Medium

Car Reflections- Screen

Reflections Quality- Low

City Glow Effect- Low Quality

World Shadow Quality- Medium

Streetlight Quality- Medium

Vehicle Light Quality- Low

Ambient Oclusion- SSAO

Bloom- On

Lens Flare- On

Classic Lighting- On

Motion Blur- On

Docked Mode (circa 60 FPS with a few dips on transitions to some areas) I have set my display resolution in steam to 1080p and have set GTA vice city to be run at 720p (1280*720). I have utilized the Steam Deck FSR to upscale it to 1080p.

Visual Effects- Medium

Display Mode- Windowed Fullscreen

Framerate- Unlocked

Vsync- Off

Anti-Aliasing - TAA+

Ansiotropic Filtering- 4x

Distance Scaling- Far

Texture Quality- Medium

Car Reflections- Screen

Reflections Quality- Low

City Glow Effect- Low Quality

World Shadow Quality- Medium

Streetlight Quality- Medium

Vehicle Light Quality- Low

Ambient Oclusion- SSAO

Bloom- On

Lens Flare- On

Classic Lighting- On

Motion Blur- On