I forgot to add that the trigger voltage on the Vivitar flashes, mostly older units, is reported to be too high for new more delicate cameras and can fry the computer circuits through high voltage on the hot shoe.

My circa 1990 Vivitar 283 works fine on my cameras, including an N90s Nikon, but I have no way of determining if the voltage is correct or not. I actually used the camera/flash combo for many years before hearing about the trigger voltage problem on the internet. Maybe I am just lucky.

Folks recommend using the Wein Safe Sync when using an older Vivitar flash on a newer camera for protection. If you have an older camera, like a Spotmatic or even a Holga, I wouldn't worry so much.