Pettine joins three external candidates in the Vikings’ search for a new DC. The Vikings are interviewing assistant head coach Mike Pettine for their defensive coordinator vacancy, according to NFL Network’s Ian Rapaport. Pettine, hired to Kevin O’Connell’s staff last offseason, joins a list of three known external candidates. Saints co-DC Ryan Nielsen and Seahawks associate head coach/defensive assistant Sean Desai have already interviewed, and Rapaport reports that Steelers senior defensive assistant/linebackers coach Brian Flores will interview on Thursday. O’Connell and Pettine first met in 2009 when O’Connell was playing for the Jets and Pettine was their defensive coordinator. Then,…