In C99 or later you can also write it as:

    Vec get_vec(void) {
        return (Vec){ { 3, 2, 1, 0 } };
    }

...or I would probably prefer designated init to make the initialization a bit clearer:

    Vec get_vec(void) {
        return (Vec){ .v = { 3, 2, 1, 0 } };
    }