Universe
In the context of this article the term Universe is used in the context of non-fictional video series. For the more convention use of the term universe see Wikipedia [1].
Most fictional work take place in real world unless there is a specific mythology and that is where you get niche universes. For example, Marvel Universe, Whedonverse and Breeniverse. They exist individually unless there is a crossover. When Marvel Universe overlaps with DC Universe I don't think it was named something new to represent the combined universes.
LG15 Universe
The LG15 Universe contains a Lonelygirl15 Universe (http://www.lg15.com/lgpedia/index.php?title=Portal:Lonelygirl15) and a KateModern Universe (http://www.lg15.com/lgpedia/index.php?title=Portal:KateModern). Some plot elements (for example, The Hymn of One) can exist in more than one Universe.
Fan Video Universe
Fan produced vidoes can exist either with the Lonelygirl15 Universe/KateModern Universe (or a combination of both) or they can exist in an entirely new universe created by the writers/producers of the fan video series. A fan video can either use elements from the Lonelygirl15 Universe/KateModern Universe or it can elements that are totally unique to its plot.