"Trees don't grow to the sky, and I see clouds on the horizon. I don't know if and when they'll arrive. A little extra caution should be the watchword," Bogle told Barron's. "If you were comfortable at a 70 percent to 30 percent [allocation to stocks and fixed income], under these circumstances you'd like to go back to 60 percent to 40 percent, or something like that."
Bogle did not believe investors for the long term should try to pull completely out and time the market, which he said is "a really dumb strategy." Instead, he said it's time "to really be thinking how much risk you want to have" and make some defensive moves.
"If I had a big liability in a year, I'd get prepared for it right now," Bogle added.
Even before the year-end extreme stock market volatility, Bogle told CNBC in April that he had never seen volatility like this in his 66-year career. For 2018, the S&P 500 and Dow Jones Industrial Average posted their worst annual losses since 2008.
Vanguard has more than $5 trillion under management.
Read the full Barron's story here.
WATCH: This '90s Jack Bogle interview shows how little his famous investing strategy changed over the years