What's all this thing that we see in films, when to break into a car you stick a long blade with a kind of hook on the end and pull up to open the door? Was it ever possible to open a car door like that? It seems like a huge design flaw, that would have been quite easy to correct. Are our cars that vulnerable?
For older cars, securing them really wasn't considered that much of a big deal (similarly, safety wasn't that important either!)
Until relatively recently, the residents of the town I grew up in wouldn't lock their cars, and sometimes would leave the keys in the ignition, so that they could be moved easily if someone needed them to get past.
So the mechanism that locked the door was not protected physically or electronically - and a slim instrument (the Slim Jim) was all you needed to get down past the rubber window seal to catch one of the mechanism rods and pull.
That's generally called a "slim jim". They certainly work on most older cars. Newer cars are more difficult, but it just takes different kinds of tools.
Slim jims are illegal in some jurisdictions (in the USA, at least), although in some states you can own them as long as you don't use them :)
If you ever lock your keys in your car, and call AAA or a locksmith, they'll likely use modified slim jims to easily get into your car. It only takes a minute or two.
Now that automobile ignition systems are so complex (with electonic keys, etc.), it's difficult for a thief to steal a car even if he gets the door open. But, if the thief just wants to take something from your car, he can always just break a window. Vulnerability is in the eye of the beholder!