In simple terms, Vega is designed to be able to easily access memory outside of their frame buffer, effectively allowing the GPU to use DRAM and even system memory to create a larger frame buffer.
In a VRAM overspend scenario, Vega is designed to keep all of the most important info in the GPU's onboard HBM memory, keeping other data in external storage, effectively allowing the GPU to use larger datasets with minimal performance degradation.
We speak about this and several other Vega features in out Vega GPU architectural analysis below.
https://www.overclock3d.net/news/gpu_displays/amd_vega_gpu_architectural_analysis/1
Vega essentially has a very smart/innovative solution for using external storage as a frame buffer with a minimal performance loss. This feature is most useful for compute applications, though it will be very useful for games that require lots of VRAM.
Imagine a user of an older GPU, perhaps a 3GB or 2GB model, who frequently runs out of VRAM in some modern games but have plenty of GPU performance. This will allow those users to get past certain traditional VRAM limitations, which will be handy for gamers moving forward.
I hope this helps.