Unfortunately, the SSL exchange has to happen before you can see clear text to trigger the redirect. You either need to use a different host name like "wwwx.yyy.edu" so your *.yyy.edu cert is usable, or you need to create a Virtual Server for www.x.yyy.edu using a *.x.yyy.edu or www.x.yyy.edu cert.
Chris is exactly correct on this. Another possible option would be to get a UCC cert which is valid for specific hostnames that all resolve to the same IP. I believe you could get a single cert valid for x.yyy.edu and www.x.yyy.edu. I don't know whether you can get a combination UCC and wildcard cert though.