While not all handsets will complain about mismatched vendors, the Sanyo 8100 is certainly particular about it. Make sure the "MIDlet-Vendor" in the JAD file matches the one in the JAR file. If there is a mismatch, the Sanyo will refuse to load the file.
You can not edit the Vendor property by hand. It must match the manifest stored in the JAR file at the time the application was packaged.