How to concatenate two numpy arrays

Two-dimensional array

a = np.array([[1,5,9],[2,6,10]])
b = np.array([[3,7,11],[4,8,12],[13,14,15]])
np.concatenate((a,b))
np.vstack((a,b))

Both concatenate and vstack return same output:

array([[ 1, 5, 9],
      [ 2, 6, 10],
      [ 3, 7, 11],
      [ 4, 8, 12],
      [13, 14, 15]])
array([[ 1, 5, 9],
       [ 2, 6, 10],
       [ 3, 7, 11],
       [ 4, 8, 12],
       [13, 14, 15]])

One dimensional array

a = np.array([1,5,3])
b = np.array([3,7,11])
np.concatenate((a,b))
np.vstack((a,b))

concatenate returns:

array([ 1, 5, 3, 3, 7, 11])

but vstack returns:

array([[ 1, 5, 3],
       [ 3, 7, 11]])