I don't know if I should jump in here, but there is some clear misunderstandings of tech going on here.
V-sync:
There are a few different scenarios with Vsync:
1) With triple buffering and DX (the most common scenario)
a) Below your refresh rate, your FPS hold strong, just like if you didn't have V-sync on, but you get varying latency between the frames
being displayed.
b) At your refresh rate, your frames get backed up, and you will always see one frame behind the most recently created one. This adds the most latency.
2) DX without triple buffering:
a) If you cannot maintain your refresh rate, your FPS takes a nose dive. You may find it wants to stay near half your refresh rate.
b) If you can maintain your refresh rate, you do not get the problems you do with triple buffering, where the older of 2 created frames is always displayed first. This has less latency than the triple buffering scenario.
3) OpenGL with triple buffering:
Unlike DirectX, OpenGL does not force every frame to be displayed, so when you reach your refresh rate, you don't gain additional latency beyond having to wait for the next refresh rate, unlike DX.
All forms of V-sync force frames to wait for the monitor to be in vertical blanking mode before they get displayed. That means there is always some latency and if your FPS cannot maintain your refresh rate, you will have frame times that jump between 1, 2 or 3 refresh times. That means your frame times will jump between 16.7ms, 33.3ms and if really bad, 40ms. This causes some stuttering.
Adaptive V-sync - is well liked by many, because it is a compromise between the good of V-sync and the good of not having V-sync. V-sync is good when you can maintain your refresh rate. V-sync causes stuttering when you can't. Adaptive V-sync turns V-sync off when you fail to reach your refresh rate, preventing stuttering, and turns it on when you are, removing tearing.
G-sync - is new, and changes everything. Instead of the GPU having to wait to send frames to the monitor to prevent tearing, or just sending it at any time, causing tearing. G-sync makes the display wait for the GPU. This means when the GPU finishes a frame, it sends it directly to the monitor, and the monitor shows it without delay, and without tearing. It removes tearing, stutter and latency.