What is ‘vast’ in an RPG? I’ve seen the adjective used mostly to describe space, so let’s take a look at sci-fi RPGs that feature space travel. Distances in the solar system are pretty ‘vast’ in human terms, but if you want a real galaxy-spanning empire, then you will need to have FTL (‘Faster Than Light’) travel — otherwise the vastness of space will simply prevent you from interacting with all of the planets and systems that are out there.
And because you have FTL and because it’s (mostly) impossible to interact with objects that are going FTL, the travel itself is not that interesting — it’s just setting a course from A to B, waiting a bit, and then you arrive. And since that’s not interesting, you just skip those parts of the story. So the whole vastness of space is just reduced, and there is actually less game time spent on travel while traversing the vastness of space than walking a few hours to the next village over in a fantasy setting.
And I’m not sure how to make FTL travel interesting. In the Elite videogames, you could be intercepted in hyperspace by the Thargoid aliens, and you’d have you fight your way out. That would be interesting… once. I’m not sure a sci-fi variant of Ryuutama would be a viable option.