Linux doing dumb nVidia things - display related

[HeeTSynC]

We've all seen that clip of Linus (Torvolds) openly flipping off nvidia for not trying to work with Linux... Actually I'm not sure who's to blame for the issue I'm having.
I have a secondary display plugged into my motherboard and the primary display plugged into my GTX1070. Everything works fine in Windows 10, even with 3 screens.
What's happening is Linux only detects the display plugged into the GPU. No amount of googling has helped me at all. CPU is a core i5-6600, mobo is Asus something (yes, I've actually forgotten and don't have the box available)

I've looked through all related BIOS settings and nothing helped, I've unplugged the secondary monitor while the system boots and plugged it in when desktop is loaded, updated intel graphics drivers, restarted countless times... I'm at a loss.
Any help is appreciated

Ps,

secondary monitor is an old(er) one, only has dvi and vga in. MOBO has both vga and dvi, graphics card has a dvi plug. I only have a vga cable and a vga to DL DVI adapter. Yes, I could get a suitable cable or adapter, or I'm thinking about buying another monitor the same as my primary and using DP in. I just want to fix it with software *if possible* before changing hardware - I'm stupid busy with work and don't have much time to go shopping for cables and such