Good news, Vigoroth is available in the games! :D
Bad news, it's post game. Vigoroth (and Slaking, on rare occasions) can only be found in the Pinwheel Forest, which is only accessible after beating the league.
You can always trade from another, completed version of either Black 2 or White 2, but for the moment I'm sorry, but Vigoroth is not available at this point. Sorry.
Hope I helped. :)