qu = (0.0, 1)T and ru = (0.5, 0.866)T ?
qu · ru = cos θ = 0.866. So θ = arc cos 0.866 = 30°.
The picture shows qu and ru and the angle between them.
Recall that qu · ru = ru · qu . The angle between the two vectors must be 180° or less (by definition). It does not matter which vector is first in the dot product.
Sometimes (especially when vectors point into different quadrants) it helps to draw a picture to check that your answer makes sense.
What is the cosine of the angle between the two unit vectors represented by qu = (0.0, 1)T and wu = (-0.5, 0.866)T ?