>We've discovered that more than one developer can open the same vcx (stored on a server) at a time. This leads to problems - when a class is modified with the vcx open by more than one person, it automatically gets removed from the library sometimes. The fix seems to be for everyone to close the vcx, someone remove it from the project, then add it back again. Anyone seen this behavior, know anything about it?

I don't know anything about it, but I've seen it too. I didn't put together that that is what was happening until just now...
Usually you can't modify the class when someone even has an instanciation of it open (like modifying a form that contains on object based on the class), but I have observed really strange behavior: Sometimes I am able to open the class for modification but the method code is all gone. Immediately close and try to edit again, and access is denied.
When a class disappears from one of our libraries, it only disappears on one workstation. It is usually enough to CLOSE ALL and then open the library or project again, and Voila, the class has reappeared.
You stated that more than one person is able to modify the class at one time, we have not found this to be true. If I need to modify a class that someone else is using, I do it in table form, or with the SuperClass utility.