How do I return a reference to something inside a RefCell without breaking encapsulation?

Instead of creating a brand new type, you can use Ref::map (since Rust 1.8). This has the same result as Levans’ existing answer:

use std::cell::Ref;

impl Foo {
    pub fn get_items(&self) -> Ref<'_, Vec<i32>> {
        Ref::map(self.interior.borrow(), |mi| &mi.vec)
    }
}

You can also use new features like impl Trait to hide the Ref from the API:

use std::cell::Ref;
use std::ops::Deref;

impl Foo {
    pub fn get_items(&self) -> impl Deref<Target = Vec<i32>> + '_ {
        Ref::map(self.interior.borrow(), |mi| &mi.vec)
    }
}