The line that's causing the trouble is this, uttered by Vaughn, as the character Ronnie Valentine: "Ladies and gentlemen, electric cars are gay. I mean, not 'homosexual' gay, but 'My-parents-are-chaperoning-the-dance' gay."
Now, is that an insensitive remark? Yeah. And I'll be the first to admit that, while I occasionally do use the word in exactly that context, it is kind of messed up; though the word in its popular context (dismissive) is seldom really directed at the gay population per se, it's still condescending, in a way, and it's still a negative term attached, however indirectly, to a marginalized population that doesn't really need the negativity. And in that way, GLAAD is probably right to want people to stop using it that way.
Here's the thing, and it's the pivotal and obvious thing GLAAD is missing here: Ronnie Valentine is not a real person. He is a character. And when there are characters in creative efforts -- even in those characters are played by real people like Vince Vaughn -- they get to say things in public that are offensive, that real people would never be able to get away with saying in public. Consider this short scene from American History X:
Is Edward Norton a racist? Is American History X "insensitive" for putting this scene in there? No. And it would be ridiculous to say so. Now, sure, there's a difference between a serious drama like American History X and a light comedy like The Dilemma, but if you look at it, the context here is almost exactly the same.
As Howard pointed out, Valentine as a character is a somewhat awkward fast-talker whose mouth often gets ahead of his brain. Meanwhile, Norton's character is a well meaning but tragically disenfranchised youth whose anger is channeled into racism. In both contexts, the scenes at hand -- though they are admittedly controversial -- are doing important artistic work: They are establishing the personalities of these characters, because those personalities, and the decisions they make, factor heavily into the action of the movie.
Public personalities should not be racist or insensitive -- and when they are, sure, let there be consequences. But the fact is, people are racist and insensitive, and for art to be effective, it needs to be able to portray people for what they are. Though Mark Twain frequently used the N-word in Huck Finn, that very text was one of the most subversive and effective anti-racist text out there for its time -- that's the beauty of art: If we portray the players as racist and stupid and insensitive and then show how they're wrong, we subvert that very portrayal.
By clamoring to have this scene removed from the film, GLAAD isn't just being whiny and PC -- it's giving itself one less tool with which to fight its own fight.