Actually, we use some 32-bit shared libraries, including OS libraries and also some 3rd party libraries, like the OpenG Zip Tools. For the MacOS beta, VIPM migrated to using command line zip/unzip tools and dropped a couple minor usability features that maybe nobody will notice (like extracting lvlib/lvclass icons from the XML file, which requires zlib deflation). However, for MacOS, we figured it was better to have things mostly working OK in 64-bit than not at all. For Windows, we wouldn't want to lose any functionality. I hear you about the memory limit issue.
I hope that's helpful information, and please know we're not hesitating to explore 64-bit, for no good reasons.