2007 and 2015 are half seasons only yes, but Vettel has participated in 9 Formula 1 seasons.
To explain my rankings of some seasons, the only point of comparison is the drivers' team mate, with the points scored against them being the strongest way to compare them. You can make allowances for mechanical failures and such if needed.
In 2006 Raikkonen did not have as big of a gap over Montoya as in 2005. When De la Rosa replaced him, Raikkonen should have been comfortably ahead, but De la Rosa actually got surprisingly close and was only outscored 26-19 despite suffering 2 mechanical DNF's to Raikkonen's 1.
2012 and 2013 were still very strong seasons, outscoring Grosjean by a considerable margin in both of them.
Seasons earlier than 2005 I didn't discuss because we were talking about the last decade only. For what it's worth 2002 was indeed poor, albeit ridden with mechanical issues, but 2004 was in my opinion his strongest season to date. 2001 and 2003 were good.
In 2009 and 2010 Vettel was impetuous, made frequent mistakes, was hopeless in traffic and only just beat Webber. In 2007 he was generally outpaced by Liuzzi, although did outscore him thanks to a great drive in China. In 2014 he was consistently outpaced by Ricciardo and was outscored badly too. By his standards that was a terrible season.