This is a good question, but it's hard to put a dollar amount on "middle class" when cost of living and the corresponding salary ranges varies so much by where you live. The term "middle class" can mean so many different things. My family lives in an expensive state, where a combined income of $100,000 would actually be considered "lower" class, especially if you're talking about a household with children as opposed to a child-free couple.
That said, "affording vacations" can also mean a variety of different things, assuming you didn't only mean Disney. Many middle class families probably can't afford a full-out Disney vacation once a year, but maybe they can afford to go all out every OTHER yea, or once every three years. Or maybe they can afford to stay at the campground for 5 days but not at the Grand Floridian for a week. Or maybe they're not going to Disney at all, and are doing weekend trips to more local areas, or renting a beach house somewhere for a week, and splitting the cost with friends or extended family. It seems to me that whatever income bracket you're in, you can find a way to make SOME kind of a vacation work, if it's something you prioritize and find value in.
My husband grew up in a house where money was extremely scarce, and they still found a way to vacation. Their vacations were driveable, and included camping or were only for a few days at a time. My family growing up was solidly middle class, and did weeklong vacations (Disney or otherwise) when the funds were available which amounted to every about every other year. Now, my husband and I as a unit with our kids, spent many years somewhere between middle and lower middle class, and we still found a way to vacation at that time. It never felt like we just "couldn't afford" to travel, we just had to wait and save until we could afford to travel the way we wanted to. However, we did make lifestyle decisions and certain sacrifices to make to make it happen.