I think in water and D2O the main factor is not Van Der Waals but rather hydrogen bonding. So the boiling point of D2O is higher than H2O not because of the London forces, but because of the difference in electronegativity between the hydrogen and the water, creating an electrostatic attraction.
This begs the more interesting question: why is deuterium's electronegativity different?
Just to clarify, electronegativity is the relative attraction an atom has for an electron.
Deuterium has a lower electronegativity than hydrogen, i.e it wants to give away its electron more. This is because the extra neutron increases the size of the nucleus and I think partially reduces the effect of the positive charge. Since deuterium has a lower electronegativity, there is a greater electronegativity difference between the deuterium and the oxygen, resulting in a stronger hydrogen bond.
Stronger hydrogen bond = stronger intermolecular forces = greater boiling point.
I don't think van der Waals is relevant here.
TL;DR: In this case, hydrogen bonding is more important than London forces
The above explanation is dodgy, I am still not sure to what extent it is true. There are many conflicting sources on the internet.
MUCH MORE INTUITIVE EXPLANATION
Heavy water is heavier, right? So it requires more kinetic energy (i.e more heat) to escape the liquid and vapourise. Hence it has a higher boiling point