Example:

use std::ops::Index;

fn test(v: &Vec<Vec<i32>>) {  
    let t = v.index(0); //ok
    let t = v[0]; //error, Vec<i32> does not implement Copy trait
}

Playground

Why does that happen? As specified in the documentation:

fn index(&self, index: I) -> &<Vec<T, A> as Index<I>>::Output

Performs the indexing (container[index]) operation

So it should be the same.