90210 actor Hood will play Dean, with Southland's Diaz cast as Kimberley, TV Guide reports.
The pair are minions of the evil Klaus (Joseph Morgan), who recently switched bodies with Tyler (Michael Trevino).
Hood - who has appeared in Emma Stone film Easy A and CBS sitcom 2 Broke Girls - will make his debut in season four's fifth episode, while Diaz - known for her roles on Army Wives and The Nine Lives of Chloe King - will appear in the following instalment.
Michael Trevino recently told Digital Spy that Klaus will "take full advantage" of being in Tyler's body when it comes to Caroline (Candice Accola).
The Vampire Diaries returns to The CW in the US for its fourth season on Thursday, October 11 at 8/7c. In the UK, new episodes are expected to air on ITV2 in late 2012.
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Watch a promo for the new season of The Vampire Diaries below: