Well, I imagined it to be some sort of level (singleplayer or multiplayer, it doesn't matter) that is centred around a vista of any sort, no restrictions there.
You can make the level as large as you want but it would be wise just to make something small and focus on the vista, it's pretty open ended I guess. That's it really, nothing else matters. Toss in a story if you like, at least like a bit of context is always good. But just ideally a vista as the main feature and a bit of space around it to compliment the vista, people should focus on ambiance, mood, detail, aesthetics and whatnot.
And my definition of a vista is a space of any sort, usually large and open that players can't walk amongst, but can see and may interest players and distract them during gameplay. For example, at the start of Half-Life 2 Episode 2, that large open space where you can see the destroyed Citadel, that's a vista, and the scene where you can see the bridge with combine walking along it in the distance a bit further in the game, that's also a vista.
@ haymaker: Ahh hehehe, you got me.