One of the reasons I find vectorworks forums and the software itself so frustrating is that I am able to troubleshoot AutoCAD, the entire adobe suite and almost any other package I am proficient in, for others, over the phone. This is because I have a lot of experience with these packages and the software always behaves consistently or at least with an identifiable logic.
I am yet to have that experience with helpdesk or the forums for vectorworks. The answer is always "upload the file", which I assume is because there is no consistency between one users issue and anothers. This is reflected in the software where a tool will either work, or not work, based apparently on the whims of the gods, or an internal random number generator.
I fixed the issue myself.. What it turned out to be was a layer with half a dozen polylines. All the polylines looked normal and appeared normal in 'object info'. However, I couldn't select more than one object without the software falling over (now that, it does consistently). I finally managed to delete these objects by deleting the whole class (??), saved the file, which immediately dropped from 280Mb to 20Mb, and purge activated immediately. I'm pretty sure this is not an issue with the files, but rather with how vectorworks builds objects. Still, its interesting to note that 6 polylines can require 260Mb of information, and make a modern computer use all its resources to select and/or delete.