Describes something extremely large in scale, area, or scope, often with a sense of grandeur. Used for landscapes, buildings, knowledge, and abstract concepts.
The national park boasts vast grounds.
He has extensive knowledge.
A vast grassland stretched out beyond the window.
That family owns a vast tract of land.
He researches the vast universe.
The professor has vast knowledge.
広大 is a more formal na-adjective for 'vast, immense', typically describing landscapes or large-scale areas, while 広い is the everyday word usable for rooms or minds too.