The Vampire Diaries, which launches its sixth season Oct. 2, has been a hugely successful series for the CW, giving the fledgling network in 2009 its highest-rated premiere ever (5.7 million viewers). Season five averaged more than 2 million viewers, which is still pretty strong by CW standards.
If all three main actors (Nina, Ian and Paul) were to sign on for a season seven, the show would have a good shot at a seventh season pickup. We also hear it's possible that the network would consider continuing the show without all three stars, shifting the focus to some of the other beloved characters.