Given a composed world of a huge number of levels where the farthest distance from endtoend exceeds the max size of a float/int64:

How does UE4 handle the computation of a distance vector between two points beeing that far apart and spanning multiple levels?

How can I compute such a distance vector depending on the answer to the previous question?
That may be up to you, honestly. Given that such huge numbers is a very special case.
You could use 128 bit values, doubles (64bit floats), or come up with a fixed math type scheme where you break the value up over two values and write special addition/subtract/etc to deal with the values.
Do you have more information on how UE4 actually handles this? To clarify: In my mind there are multiple distinct levels with independent coordinate systems. When requesting a Vector spannign multiple levels there must be an additional “master” coordinate system or a single vector for each sublevel is calculated based on their respective entry/exit points.