How to get 1 GB VRAM

if a particular piece of software is telling you that you need 1GB of dedicated video RAM from a Graphics Processing Unit then that particular software may require something more sophisticated than the integrated Iris card to run.

the GPU vs Integrated thing becomes an issue with high end 3D software, video editing software that might utilize a technology specific to one type of card vendor (e.g. Nvidia's CUDA, and ATI's RenderPro) - Apples software is generally specific to existing Apple hardware so it's less likely an issue.

Most if not all game software will run with a dedicated GPU or intergraded Iris if there is enough RAM. The GPU RAM is on the card and the system can not utilize it. The intergraded cards from the same era as dedicated GPU's may have far more limits, they share system RAM and it can be much slower than the RAM on a GPU affecting video performance.

1024 Mb = 1Gb

Integrated video on Intel-based Macs - Apple Support