vsync?

no, it just syncs fps to ur monitors refresh rate
v sync on with 120hz -> max fps 120

Yes but dunno why :)

vsync is bugged on et, don't use it.

generally speaking vsync can induce a bit of lag if:
fps is higher than the monitor frequency
fps is higher than the monitor frequency and you have triple buffering enabled
fps is lower than the monitor frequency (feels like it's skipping frames, and actually it is), triple buffering helps in this case.

a possible 'workaround' to avoid this lag some people do is to limit the fps to the same value as their monitor frequency (60fps/60hz or 120fps/120hz) assuming the fps will never get below the monitor frequency.

vsync can be tricky, specially if you are planning on using it on a multiplayer game, which i don't recommend, i only use it on single player games where being the possibly fastest doesn't really matter.

vsync adds lag to your mouse, and it eats fps.
you could reduce the fps loss by using tripple buffering, but you'd add even more log to your mouse

dont use it for fps gaming

120hz there is no effect.

If you play on 60/75hz monitor it will lock your fps to 60.