What is your screen refresh rate? With Vsync on, you are capped to the screen refresh values. If you have a 144Hz screen and you're running 165FPS with VSync off, you will run 144FPS with VSync on. But if your GPU struggles to hit 144FPS, with Vsync on it will drop instantly to 72 FPS, then to 36 FPS (assuming no GSync/FreeSync).