Managing memory is one of the tasks software developers must do to write stable software. Automatic memory reclamation systems do not always work properly.
I notice that when VZAM runs for many hours or days, the amount of system memory used slowly increases. After I exit the the program, the memory use drops quicky to "normal" levels. For example today my system reported that I was using over 1,500 MB of memory, after exiting and restarting VZAM, the memory use dropped to 500 MB. So over 1000 MB (1GB) was returned to free memory.
VZAm is not the only program with these problems, IE and other browsers, are really bad as well - their excuse is that that "must" obey commands from websites for more memory - I don't buy that. Of course, windows is much better than it was since it dumps "lost" memory on to the harddrive "virtual memory" file (which can be huge). So you may not see a crash, if you run things for days on end, but stuff is happening in the background. Of course, Apple is not immune to these kinds of things as well.
Now, VZAM is pretty simple compared to a fully featured web browser so they should be able to fix this problem. For now, the workaround is to exit and restart the program at least once per day, when convenient. Disconnecting does not reclaim that memory - it must be an exit.