Are those video cards of a Mac even available on PCs unmodified? If there's no drivers and/or compatibility for the cards in PC software, it's useless anyways?

Utilizing real hardware from virtualized/emulation environment is not an easy feat, unless the underlying hardware generate no interrupts or DMA operations, which isn't the case for video cards. Several technologies have been available, such as SR-IOV, IOMMU, EPT etc. to assist software to do so for x86 systems. The Mac ecosystem is no longer centered on PPC, so I doubt it will get any of such technologies to enable virtualization/emulation to use real hardware.

You mention VPC, 3D games and Windows so I'm assuming you are talking about trying to run Windows 9x x86 D3D/OGL/Glide games on a PPC mac.

CPU would require alot of overhead so emulation of the GPU wouldn't be usable. Hardware doesn't support vt-d so no passthrough.

The only possibility would be by utilizing a wrapper but you'd have to determine what the minimum PPC CPU would be needed as well as the minimum video card for playable performance and then either code a custom emulator or more likely utilize DOSBox or qemu with some modifications.

IIRC, VPC 3.0 emulated a Voodoo card back in the day but it was far from ideal.

Really if you have to play x86 3D games on PPC you'd be better off with someone designing a PCI card with the CPU and video card built-in like they did back in the day but if you're going down that route you may as well just use a PC.

Judging by how weak the PPC Macs are when using them to emulate DOS, emulating more is really asking for a slideshow.