For most of the time Vettel was performing very well throughout 2008-2013 and that's a long time actually. That's more than being a one-trick pony, because regulations changed a few times during that period. As to why he is struggling in 2014 is another matter - perhaps there is a certain adaptability window for him, as in some things he can't go that well. Räikkönen was performing well up-to-and-including 2007, so that period also included many years, but suddenly hit a ceiling in 2008 and dropped backwards. So these things can happen.