For those who want to test the triggering voltage for themselves, you only need a standard voltmeter, set to 300 volts DC. You can test the flash as follows:
Turn the flash on.
Touch the red (positive) lead to the center contact in the middle of the flash hot shoe (Photo #2).
Place the black (negative) lead on the metal base of the hot shoe (Photo #1). In this photo, I measured the voltage on a Yong Nuo 560 flash. It is a very reasonably priced, fully manual flash that can accept an external battery pack and has a supplementary PC terminal. It has no provisions of non-TTL exposure automation. As you can see in Photo #1, the Yong Nuo has a triggering voltage 3.27 volts, well below the 6 volt ceiling for the Canon.
The generally accepted guideline concerning Vivitar flashes is based on where they were made. In the case of the Vivitar 283, assume that all Japanese editions have high triggering voltages, and are therefore unsafe. Conversely, non-Japanese models are considered safe, but check just the same. The same applies to the 285, although all releases of the 285 HV is supposed to be safe. Still, check the voltage. And cross your fingers.
I can’t (and won't) guarantee that this test will prove that your flash will be totally safe with your digital camera, but it is useful procedure to know. If you go to YouTube, you can find a number of film clips showing other users testing their flashes in exactly the same way.