Almost nothing in After Effects uses the GPU and VRAM. After Effects does nearly all processing on the CPU.
See this video for an exhaustive account of what After Effects does with OpenGL on the GPU:
Go here for lots of information about After Effects and performance.
In the video the narator says that Open GL shouldn't be used for final output because of lack of fidelity. What does this mean? It renders inaccurately?
Yes, the OpenGL renderer is inaccurate. It's fast, but it's also inaccurate. That's why it's good for rough previews but not for final output.