def vector_element(r, i, j):
    r"""Extract an matrix element of a vector operator.

    >>> r = define_r_components(2)
    >>> vector_element(r, 1, 0)
    Matrix([
    [x_{21}],
    [y_{21}],
    [z_{21}]])

    """
    return Matrix([r[p][i, j] for p in range(3)])