Seconded. Helicopters are hideously vulnerable to ground fire, as the U.S. learned to its chagrin in Vietnam and the Soviets re-learned to their chagrin in Afghanistan. Even armored helicopters like the Mil-24 can be (relatively) easily brought down by ground fire from heavy machine guns. The early helicopters didn't have that much of a payload, so the amount of armor they could carry was minimal.
As some one who was in that position, I can tell you that your partially correct. There were times where my aircraft looked like Swiss cheese, but it made it home. While another instance one round brought the aircraft down. It all depended on where and how the rounds hit the aircraft. As for armor, only the pilots in the Huey's had armored seats. The flight crews had body armor, consisting of a of a hardened front and back plate.
It was found though that the back plate did more damage then it prevented. Since the rounds generally came at you from the front a round that came in at such an angle as to miss the front plate, it would bounce off the back plate until it hit the front plate from the inside. So crews removed the back plate and sat on them. The biggest difference between aircraft and helicopters is a simple one. You can't bail out of the helicopter!
Kind of think of Helicopters as the WWI aircraft of the modern air war!