but my understanding is that vert sync is for when you are consistenly getting over 60 fps (or 30, I suppose).
This isn't quite true. Here's how it works (more or less):
Your monitor is quite likely running at 60 Hz. This means that if your computer sends it 60 frames per second they will all render perfectly. The same goes for 30 FPS, where each frame will stay on the screen for two "frames" seen from the monitor's perspective. You can keep halving this value. If running without vsync, any deviation from these exact values means a frame may start drawing before the last one is finished, showing a mishmash of two or more different frames on the screen at once. To combat this phenomenon you can enable vsync which locks your framerate to the ones mentioned above. The monitor will display the amount of frames per second equal to the first lower
value below what you're capable of transmitting. This means that if you're capable of rendering at 70 FPS, you will be locked at 60. If you dip below 60 you'll suddenly be locked at 30 FPS until you can get above 60 again. The same goes for dipping below 30. The result is that you risk getting dramatically lower framerates than your computer is capable of handling when you turn vsync on, but the picture will remain clean and you will also prevent your graphics card from rendering way too much data during less intensive scenes, which could lead to overheating. You will also experience much more obvious dips in framerate during high-intensity scenes. If you're hovering around one of these exact values, vsync may become jarring as a dip of 2 FPS could end up being an actual dip of 30 FPS once the lock takes effect.