When Vsync is activated, then the video is flicking on the output while "on air"
Verify that the refresh rate set in Smode is coherent with the refresh rate of the monitor on which you have your output.
First go inside Advanced display settings, under "display adapter properties" and "Monitor" to the the actual refresh rate of the monitor on which your output is set.
Then in smode, go inside Edit->Preferences->General to configure the refresh rate.
Then go inside Edit->Preferences->GPU Settings to activate the Vsync.