What is more important number of CUDA cores on GPU or VRAM for video editing and graphics?
It depends entirely on which software you are using, as well as plugins. Adobe Premiere takes advantage of CUDA, which radically increases rendering speeds and playback of specific file types. 

But if you've applied filters, or 3rd party plugin filters that do not take advantage of CUDA optimization, your bottleneck will be the speed of your CPU. 

Alternatively, certain 3D platforms are only accelerated by NVIDIA cards, others, require AMD. 

The amount of VRAM affects how much data the card can handle in one single load (typically 1 frame), which comes into play mostly with 3D work.