I've upgraded the GPU on my Mac Pro 5,1 from stock GPU Radeon HD 5770 to NVIDIA Quadro K5000 in order to be able to upgrade to Mojave which is officially the last supported Mac OS by Apple.

I soon figured I can upgrade to Big Sur by using OpenCore so I went ahead and successfully did the upgrade without too many issues. The main problem I'm having now is that I'm using a 3 monitor setup. 2 of the monitors are Dell E2414E connected via DVI and the third monitor is a cheap 20" Dell Monitor with VGA support only. The problem is with the third monitor that's connecting using VGA. For some reason it seems like the GPU doesn't want to play nicely with that monitor. The weird part is that on startup, all three monitor receive signal from the GPU and sometimes the Apple logo shows up on the VGA monitor but as soon as the screen goes to the loading bar, the VGA monitor turns off and only the 2 Dell Monitors connected via DVI are the ones that remain working.

Specs:
GPU: NVIDIA Quadro K5000 4 GB
OS: Big Sur 11.5.1 (20G80)
Processor: 2 x 3.33 GHz 6-Core Intel Xeon
Memory: 20 GB 1066 MHz DDR3 (4x 4GB, 4x 1GB)

I've tried several things after reading online forums like resetting the SMC and NVRAM but no luck. Hoping that someone out there may have ran into the same or similar problem and could advise on a workaround for this. I also read online that Big Sur doesn't support VGA all together but in this case why would the boot up screen show up then disappear?