gasaraki :
What Wildcard said is counter productive. v-sync locks frames to the refresh rate of the monitor or half of it or a quarter of it. So if you are not getting 144fps, vsync will drop it to half that. Don't use v-sync on a 144Hz monitor IF it doesn't have Freesync or G-Sync.
FPS isn't fixed. It varies depending on the scenery and what's going on the the game (how much the GPU has to render).
Double-buffered V-sync could cut your framerate in half worst-case. But with the copious amounts of VRAM available today, nobody should be doing that anymore. The extra VRAM for triple-bufferred v-sync is less than 8 MB for 1080p, 32 MB for 4k. Triple-buffered v-sync doesn't affect your framerate at all, it will only add a slight lag (max of 1.999... monitor refreshes worst case). All v-sync does is delay displaying a frame until it's completely drawn. Say your GPU can render only 140 FPS but is displaying on a 144 Hz monitor.
■Without v-sync you will get 144 "different" frames per second, but parts of those frames are the same as the previous frame. The line where an old frame image switches to a new frame image is a tear. Because of the framerate varying, this tear line appears at more or less random positions. But if your FPS stabilizes at an integer ratio of the monitor refresh rate, the tear line can appear in the same spot each frame (which makes it easily visible). If the FPS is slightly different from an integer multiple of the refresh rate, the tear line will slowly travel up or down (which is also easily visible).
■With triple-buffered v-sync the GPU is producing 140 FPS so you only get 140 FPS, and 4 of those frames will be displayed twice to make 144 FPS, thus matching your monitor's refresh rate. There are no tears.
There's another possible issue called jitter (animation may not be as smooth as it could be). Whether v-sync has more or the same jitter as no v-sync depends on which part of the screen you happen to be looking at at that moment. But since the entire frame is affected by jitter in v-sync, while only part of the frame is affected with no v-sync, it can be considered worse with v-sync enabled. That's really the only issue you'll face if your FPS is less than the monitor's refresh rate. (It also happens when FPS > refresh rate, but because it's happening at the refresh rate instead of via duplicate frames, it's less noticeable.)
Freesync and G-sync make the monitor wait until the GPU finishes a frame before it refreshes. Basically they're monitors with a variable refresh rate.