I use both a Metz 54 and a Vivitar 283 with my M8.
As implied above -- MAKE SURE you use a Wein safesync or HSHS adapter to keep excess voltage from reaching the M8 -- when using the Vivitar.
You can see voltage (dump) values here:
Photo Strobe Trigger Voltages
and the Vivatar 283 has been reported as dumping as much as 600 V back into the hotshoe.
My method is to use a sensor in the hotshoe (with HSHS if it's the Vivitar sensor) with an extension cord and to set the Vivitar to Auto and the iso to the camera's setting. If it's the Metz, the extension cord reads everything except the aperture from the camera.
For both flashes, I use f8, 1/250, and iso 160 if possible.
The Metz will do something called GNC that is supposed to approximate TTL-flash. This is because the M8 can't do TTL. I don't use the GNC feature for people because a pre-flash is used by the camera to calculate a necessary strobe output, and I find that pre-flash generally ruins portraits. The subjects see the first flash and go somewhere else in their minds.
I find the Metz flash fantastic. It brings MUCH more POWER to the equation.
Regards,