DirectX 11 is sometimes your only choice if you want to play splitscreen couch co-op on the PC.
Vulkan results in many graphical glitches and certain areas are unrenderable (the culprit being the black boxes so many people mention). My partner and I reached a point where we couldn't move on anymore and graphics drivers were already updated and game was already updated so we were confused as how to move forward.
After searching for a few hours, I found a post specifically about the splitscreen and how this seems to affect lots of people trying to play splitscreen with Vulkan.
I personally prefer Vulkan and think it runs better in single player mode, but if you want to play splitscreen without major issues, I strongly suggest DirectX 11.
I've been using Vulkan in Linux with an AMD card. Seems mostly fine except the occasional black boxes during cut scenes (about 15% of the edge of the screen). I haven't tried DX11 yet.
I'm also in Vulkan on Linux with an AMD card. I don't get those black boxes.
The main menu has terrible framerate, but everywhere else is acceptable through Proton (45-50). DX11 has great framerate on the main menu, but like 8-10 FPS ingame (my Windows partition can hold a steady 60).
Yes which is why I chose Vulkan over DX11. But depending on the Vulkan implementation for a specific game, sometimes converting DX to Vulkan might function better.
I'm on Linux with AMD, but Vulkan is a crashy mess for me. Can't keep the thing running more than a few minutes. It's fine on DX11 (a few stutters here and there, but hasn't crashed).
Running on Windows 11 with a 6900XT and Vulkan and DX11 look effectively identical as far as I can tell but Vulkan runs a lot faster and doesn't stutter when loading a new area.
Unfortunately though it tends to crash on tab out which means I need to use DX11 because I play with friends.
Yes. It's not really meant as a visual improvement but as an efficiency improvement. Sadly it does seem (for some, myself included) that the Vulkan build needs a bit of work. For me it crashes all the time, and Larian themselves mentioned that it isn't quite as stable as the DX11 build.
I tried to play on Vulkan with my Arc A770 (Arch Linux) and it just black screened. With DX11 it played at around 60fps with VRR (dips down to the 50s) at 1440p.
Linux with 6.1 kernel and Kisask-Mesa drivers on an RX 6700XT: Vulkan works better with fewer crashes/glitches for me. Also seems to load a little faster, but that could just be because I've used it more so there's more cached shaders.