I don't know of any site that lets you actually download and/or watch full episodes of vampire diaries online BUT if you do the Netflix free 30 day trial thing you can watch Vampire Diaries free. I'm watching it through Netflix right now and it is AWESOME!! :) link
You can search the show in Youmovieset [.] Com
and once you get in the website you will get the show Vempire Diaries along with other shows. I hope it helps