I did some investigating into the flash shoe interface on digital cameras.

There is an ISO standard that the flash shoe adheres to for al cameras,
but...

Film cameras usually use a mechanical switch to fire the flash. The switch
can handle several hundred volts, over the years it will become corroded and
pitted from the spark produced at each flash.

In Digital cameras, the switch is an SCR or transistor and some camera
manufactures claim that attaching a flash that has more than 12 volts on the
contacts can damage the camera. I took out my voltmeter and measured one of
my manual flash units as having a 65VDC switching voltage and the other at
6VDC. Just because it fits, doesn't mean it will work properly or not cause
damage.

Some cameras such as my Nikon 5700 do not control the flash through the lesn
systems, it is doen with a little sensor near the flash itself. Witht he
proper Speedlight attached, the camera will use this sensor to tell the
flash that sufficient light was produced for the image being taken. Because
there is some minimal interaction with the camera, the camea may recognize
an external flash has been connected and therefore not fire the internal
flash. With my manual flash on my camera, the cmaera doesn't know htis, so
it always fires the internal flash and the external flash fires as well, it
uses it own sensor for the scene. Thus, the two seperate systems can get
confused. For instance, if the internal flash fires first, and it's output
is sensed by the sensor on the external flash, the external flash may not
provide as much power for the flash as expected.

You may have to provide a cone or shielf around the Vivitars light sensor to
prevent it from picking up the internal flash and being fooled.

rtt

Always check your flash switch voltage before connecting it to yur camera