The Texas Chainsaw 3D star will play Aaron, who meets Elena (Nina Dobrev) at college, E! Online reports.
On the run from a dark past, recurring character Aaron will bond with the tortured vampire.
The 31-year-old Sipos previously starred on The CW's Life Unexpected and also appeared on the network's short-lived Melrose Place revamp.
Vampire Diaries star Ian Somerhalder recently told Digital Spy that he is "not thrilled" about new spinoff series The Originals, since it takes "amazing actors" away from the original show.
"Who are the other people who are gonna be coming on the show? Are they gonna be good?" he posed. "Because it's The CW, and everyone has to be pretty, and there aren't a lot of pretty people that can act well.
"I'm one of them, that can act well. Not to sound s**tty about it, but casting is always tough."
The Vampire Diaries returns to The CW on Thursday, October 10. The fifth season will also air on ITV2 in the UK.
> Vampire Diaries casts True Blood's Janina Gavankar
Watch the Vampire Diaries Comic Con sizzle reel - featuring footage from season five - below: