I am not quite sure how is MemoryBreakpoint related to extracting Vector
Anyway, yes, it is defined/required in debug.
It works like this: Each allocation with U++ allocator has a serial number. If you leak (or heap gets corrupted), you are provided with this serial in the .log.
You can then set MemoryBreakpoint, using this serial, to catch the source of the leak (it segfaults when it is done, so you can catch it in debugger and see backtrace). In App.cpp it referenced because you can setup MemoryBreakpoint using commandline.
Anyway, I think that for extracting Vector, it is mostly irrelevant.