I think you might be confused, friend. The Vice City Next Gen project is standalone. Meaning it never required GTAIV to function properly. You install it and it’ll work by itself. Treat it as it’s own separate game.

These are my specific methods to get the game working- separate from the OP. I used Bottles instead of Lutris to sandbox the game.

I eventually got it working under very strange conditions. I had to grab the xinput1_3.dll from the Internet and tell Bottles to set it as one of my dll overrides.

The game now boots fine- but the graphics WILL corrupt every time you load the game. Changing certain settings (like messing with the Reflection quality or the resolution) fixes the graphics.

It’s a pretty annoying issue atm, but at least I can confirm this game does work on Steamdeck via Bottles.