Vo2 of the brain is 21% of Q=13% or of the entire blood?
TL; DR 21% of the whole body.

The VO2 numbers add up to 100%. You are looking at the way six major compartments of the body split the oxygen consumption. The brain takes 21% of the whole oxygen brought into the body by the arteries.

The Q numbers add to 100% too. The brain takes 13% of the blood coming out of the heart.

The picture is meant to contrast high-flow and high-metabolism organs. The kidneys take 20% of all the aortic blood, but use only 7% of the oxygen consumed in the body. The coronaries take 4% of the blood leaving the ventricles, but use a large 11% of the oxygen take in by the whole body.