You probably want to show as much of a highlight of your final product as possible, so I would say you want to take a cross section, rather than focusing on one part at the exclusion of others. So you would want to focus on the overall feel of what you are trying to create, even if it is only 1 hour of that feel. That said, yes, you can put out a game that is just a prologue with no defined ending, or a middle chapter for that matter.
Viewing post in Clarification on completeness of the game.
https://itch.io/t/317351/judging-criteria
The judging criteria gives a good idea of what kind of stuff we are looking for. A vertical slice is probably going to hit on all those things better than a rough prototype will.