What is the aerodynamic reason that the An-225's vertical stabilizers are not simply vertical?
I'm fairly sure (but have no sources to confirm) that the reasons are purely structural. The vertical stabilisers are simply set at a right angle to the horizontal one. This is just the easiest thing to do, and it also provides better clearance with deflected lower rudders. The aerodynamic consequences of this (for such small angles) are pretty minor.

Now there are good aerodynamic reasons to have dihedral on the horizontal stabiliser (which causes the whole tilt), but this is a different question, and it applies to many airplanes.