The parens were not unbalanced, that would very likely not even compile. They were misplaced, a much more subtle mistake.

Doesn't need a plugin: http://stackoverflow.com/questions/232274/how-do-i-get-vim-t...

There's also some info and some plugins listed here: http://vim.wikia.com/wiki/VimTip630

Although, a misplaced parentheses could still be balanced. For example,

    if ((!x < y))
        ...

versus

    if (!(x < y))
        ...