It’s slightly more complex. The “FPS” in the VSync section in FS graphics options is really based on an assumption of a figure of 60Hz refresh rate so “30” is actually meant to represent 1/2 the refresh rate which for 60Hz is 30FPS. So if you have a 75Hz monitor and set it to be 75Hz in NVCP then choosing “30” under the VSync section will yield 37.5 FPS (1/2 of 75) and “20” would yield 25FPS (1/3 of 75Hz). It’s badly labelled, and only numerically true if refresh rate is 60Hz.
Explained better in this post
So in answer to your ? You could set 75Hz RR set in NVCP and “30” under VSync which would give you a locked 37.5FPS OR set 60Hz in NVCP and “30” under Vsync in game, and then it would be locked at 30FPS.
Try both out and see what’s smoothest with your hardware.