Hi, the Vector skin for Wikipedia is not enabled on Wikia. Wikia uses the Oasis skin as the default for all users and you are not allowed to change that. (You can switch to the Monobook skin in Special:Preferences, but that only affects your single account. You cannot change the skin of the entire wiki for all users, that's impossible.)
Back in 2010 when Wikia switched to the Oasis skin, they allowed a few special wikis like Uncyclopedia to have custom skins. (In the case of Uncyclopedia, it's because they parody Wikipedia and so their wiki needed to have a Wikipedia-like skin.) That was a special, one-time provision that only applied to those few select wikis in the past. All wikis since then are required to use the Oasis skin.