In the comic books were the glasses just part of the disguise, or did they actually correct his vision?
There is nothing wrong with Clark Kent's vision. The glasses are merely part of his costume.
In the earliest Superman stories, Clark Kent was mostly just an excuse to show what Superman did when he wasn't being Superman; everything about the Clark Kent persona was designed to be "anti-Superman". This extended to his wardrobe, which was as low-key, nondescript, and non-threatening as possible: business, suit, tie, glasses. The only thing special about his glasses is that they were made up of special Kryptonian crystal that would stand up to his heat vision.
Early on, Clark's glasses were given some other odd disguising properties, though none of them had anything to do with his vision. At one point I believe the glasses did something to make his eyes appear a different color or shade of blue or something; in the 1970s, as @Richard points out, there was a very short period of time where his classes were even mildly hypnotic. All of these were meant to help explain away the fact that Clark Kent otherwise looked just like Superman, but no one ever noticed.
In later stories, the character development changes, but the basic idea remains the same. Although modern Clark Kent is a far more fleshed-out character -- in fact, many modern Superman writers consider "Clark" the real person and "Superman" the alter-ego -- the fact remains that Clark wears glasses simply to disguise himself. He is typically not shown as wearing glasses until after he comes into his powers and begins operating as Superman.