OK, well I seem to have resolved this. For some reason IE on the server doesn't show a copy of the proxy script, but I pulled one from my local machine and sure enough, the server uses a different proxy server. It still didn't work when I configured it, don't know why, so I tried creating a task that would just make an HTTP connection to the VisualCron update server, so that I could see in more detail what was going on. Once I got past the proxy server with that, and got VisualCron's "unauthorized" error because I didn't have its login credentials, I put everything back in the VisualCron settings proxy information, and now it works. Go figure.
Rebeccah