[Bonnie]

Had to look this up, never heard of a. eating this combo, or b. said combo being bad for you. Apparently, your mom's not the only one to think this, must be an old legend? someone else asking this

I agree with them, it's probably because vinegar will curdle milk, so at some point a long time ago someone assumed it would do that in your stomach. But as mentioned earlier, since there's already acid there, I doubt it would.