Presumably, your grandmother has two daughters and a son (your father), and has decided not to include any grown men on this family vacation. Maybe your father is thrilled about this, but if he's in the picture, and has a decent relationship with his mother, it would be helpful if he could be the first line of defense with his mother for discussing this slight against you and your daughter (and his wife, if she's at all inclined to be included). You're right, it doesn't sound as if Grandma is malicious, just deeply oblivious to the fact that she is gassing on about her wonderful trip with her other granddaughters to the excluded granddaughter. If your father won't intervene, then you should speak up. When she next mentions the preparations or wants to show you the cute snapshot of little Ashley with Mickey, tell her, "Grandma, I love you, and it hurts very much that I, and my mother and daughter, have not been asked to join the rest of you on your annual vacation," and see what happens.
I'm a third-year medical student at my home state's medical school. I went to an Ivy League college as an undergraduate, then worked for a few years before going back to school. Inevitably in conversation, fellow students will ask each other where they went to college, and when they hear from me, the response is almost always the same: "Oh, don't I feel stupid now," or, "I better be careful about what I say around you." Most of my peers attended the state college or local schools, but my feeling is that we're all now at the same place, so why make a big deal about it? I don't advertise where I went to college, and attitudes only change after I answer this particular question. I don't want to be snarky or defensive, and it's come up enough that I'm getting tired of the whole thing. Can you suggest a few polite rejoinders that I could use, because I truly hate these situations.
Unfortunately, your classmates are exposing their own insecurity about something that's ridiculous—and by the third year of medical school, haven't they all started thinking of themselves as gods? Almost any rejoinder is bound to make things worse. You don't want to say something humble that actually implies, "Yes, I have an Ivy League diploma, but I'm really just average like you," or, "Don't be jealous, my superior undergraduate degree doesn't really mean anything to me." Sometimes just a shrug, a smile, and a change of subject ("Did you see the size of that polyp on the guy in room 317?") is the best approach, particularly when someone is daring you to start pouring gravel on that chip on their shoulder. You've been together for three years, so they know you don't flaunt your degree or have an attitude about it—do your best to ignore theirs.