IBM did offer an 8-bit VGA card, but it was targeted to the PS/2 Model 30.
However the card does still work in a 5150/5160.
There was a discussion about this card in another thread here:
The IBM 5175 PGD monitor was very close to VGA, and used the same basic case as the 5153/5154.
There was a small circuit mod you could make in the 5175 to allow it to run from a VGA card.
Isn't VGA 100% compatible with EGA? I haven't encountered any problems running EGA software on a VGA system, unlike how some CGA software will usually not work on VGA (such as the hacked 16-color 160x100 graphics mode).
It depends on what you mean by "compatible". At a BIOS level, sure. But at a register level, close, but not exactly. But it's good enough for almost anything.