The four-time world champion will leave the Milton Keynes-based outfit at the end of the season, to be replaced by Toro Rosso's Daniil Kvyat.
However, Ricciardo has suggested that the chance to drive at a rival team may have been an opportunity that was too good to turn down for the 27-year-old German racer.
He is quoted by the Daily Mail as saying: "I expected him to leave, so I wasn't shocked when he announced he was leaving.
"The way I see it, he has won four titles with Red Bull and obviously he has an opportunity elsewhere.
"Before he sees himself getting a bit too old for the sport, he probably wants another challenge."
Ricciardo is currently third in the drivers' rankings, 56 points ahead of Vettel.