VOES does not retard timing. It advances it. Have you ever heard the term vacuum retard? No, it's because you hear vacuum advance. Which is all the VOES is. When a VOES fails, there is a spring in it that will cause it to fail in the open position, a failsafe if you will. The one time I have had one go out, the silicon in it basically ran out. This created a huge intake leak, and my bike wouldn't do anything but idle. As was said they can be tested with a vacuum pump and a multimeter. Set it on continuity (Ohms) and beeper if you can. Hook one lead to each wire from VOES (don't matter which). Pump up vacuum pump and let air bleed off. Multimeter will be beeping until the switch separates (because the spring overrides the air vacuum). If it quits beeping at 4.5, then you know the VOES is for a light bike, like a Sportster or FXR. If quits at 7, you know it's for a bagger. Anyways, this is a very easy test to do. I would recommend it before you go buying another switch because they are around 60 bucks.
Read this and it will help you understand it.