Are fighter jets vulnerable to .50 caliber gunfire?
By .50 caliber, I believe you are referring to the .50 BMG cartridge.  The answer is yes, you could shoot down a jet fighter using such a round and plenty of jet fighters, such as MiG-15s, have been felled using that round during the Korean War.

In some respects the .50 BMG is a superior round to modern cannon rounds for shooting down jet fighters.  It's a small, fast round with good range, muzzle energy and penetration capability and can be fired at high rates, saturating sections of the sky with rounds, particularly useful for high off-angle snapshots in ACM.

In regards to penetration power, even a .22LR round could punch through the skin of a modern fighter.  The structure has to be kept light for weight and performance reasons.  Armor on fighters is almost unheard of, save for some plating on flight crew seats.  One exception to this is ground attack aircraft such as the A-10 or SU-25 which do have armor around the cockpit to protect the pilot at low levels.  The rest of the aircraft does not have armor and relies more on structural integrity and redundancy to protect the aircraft from ground fire.