There's reason to be concerned about bond vigilantes, who are no longer under "lock and key" and are free to push yields higher, Wall Street veteran Ed Yardeni told CNBC on Friday. Yardeni, a market historian, coined the term bond vigilantes in the 1980s to refer to investors who sell their holdings in an effort to enforce fiscal discipline. Having fewer buyers drives prices down — and drives yields up — in the fixed-income market. That, in turn, makes it more expensive for the government to borrow and spend. "They had been sort of put under lock and key by...