Vector is a non-standard Wikia skin and the skin used on Wikipedia. It is not available as choice in Special:Preferences. As Monobook is however somewhat similar to this skin, it may be customized to look more like Vector as well. On MediaWiki, Vector is currently the default skin.
Vector looks different from the standard Wikia skin in many ways: the navigation with helpful links is placed on the left side, links to user page/talk/contributions are placed in a row in the top right, with the search function right below them, and the article takes up the widest possible space with edit/history/etc. tabs at the top.
Notable differences with Monobook include the placing of the search function, that is placed in the navigation on the left in Monobook but right above the right side of the article in Vector. The Edit and View history tabs above the article have also been moved to the right side, while on the left side remain the Page and Discussion tabs. Additionally, the bottom rail with the license and the like is not a separate area.
Wikis using VectorEdit
The following are well-known wikis that use vector: