FREESYNC WITH VSYNC
If vsync is enabled, it is only active when the FPS is above or below your monitor's refresh rate range.
If FPS is below, the monitor has no choice but to use vsync in the double or triple-buffer mode you've set. This will avoid tearing, but add input latency.
If the FPS is above, the GPU will reject frames ("FPS cap") to keep the application inside the FreeSync window. It will enforce smoothness. You won't get the lowest possible input latency due to rejected frames, but no extra latency is being added.
When your game is in the FreeSync window, this is the lowest possible input latency.
FREESYNC WITHOUT VSYNC
If you really care about input latency, then you can turn vsync off.
If the app is inside the FreeSync window, FreeSync is active. This is the lowest possible input latency.
If the app is below the FreeSync range, monitor will run at max refresh until the app's FPS gets back inside the DRR window. You will experience tearing, but no frames will be buffered or held as with vsync.
If the app is above the FreeSync range, monitor will run at max refresh and your FPS can go however high it will. This sustains the lowest possible input latency because no frames are being buffered, held or rejected as with vsync. You will experience some tearing until the FPS falls back inside the FreeSync window and FreeSync resumes.
IF YOUR MONITOR IS COMPATIBLE WITH FREESYNC LFC
If your monitor has a sufficiently wide range to support our Low Framerate Compensation feature, this supersedes your vsync setting. It has lower input latency, no tearing, and no vsync stutter. It's much better than vsync.
Most people want to leave FreeSync + Vsync enabled.
PROS: GPU won't waste power/heat/noise on unused frames, game forced inside the FreeSync range as often as possible, no stuttering, no tearing.
CONS: Lowest possible input latency will not be achieved if app goes outside of FreeSync range. Vsync stutter possible when app is below FreeSync window.
If you're a stickler for mouse latency, use FreeSync + vsync OFF.
PROS: No stuttering/tearing inside FreeSync window, lowest possible latency at all times.
CONS: Tearing possible when app leaves FreeSync window
IF app FPS < min_refresh THEN Low Framerate Compensation (LFC) supersedes vsync.