My Multi Theft Auto client doesn't detect VRAM properly, how could I solve this?

I'm trying to play Multi Theft Auto: San Andreas, but the custom GUI can't render most of the time, because it says, it's out of VRAM. As you'll see the detected VRAM is around 250MB, but I have 6GB, which should be sufficient for this game. All my drivers are up-to-date.
What could I do, to increase the amount of detected VRAM?
Edit 1: Using the built-in scripting interface, I checked, that the used GPU is my discrete GPU ("NVIDIA GeForce GTX 1660 Ti with Max-Q Design"), and the detected VRAM is 256 MB.
Edit 2: I suspect, that the game can't work with cards, that have more than 4 GB vRAM. Could this be true? Can I temporarily decrease the capacity of my vRAM?
Debug information:
MTADiag
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MTA Problem (kicked from server)
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