Why do molecules vibrate?
Because the lowest temperature we've accomplished for molecules is not quite
Just as people breathe because breathing takes in the oxygen from the surroundings and cyclically incorporates it onto hemoglobin (so that eventually we breathe out
...molecules vibrate because vibrations take in the energy from the surroundings and cyclically incorporate it into constant natural vibrations (again, and again, and again).
If a molecule did not vibrate, there is much less the surroundings can transfer energy into, the human-constructed law of conservation of energy would no longer hold true, and energy from the universe would be in excess.