vsync is limiting fps to refresh rate, so if you get it limited to 60 you will have 60Hz refresh rate and that is bad for eyes if you use CRT monitor, I have 85Hz refresh rate so vsync limits to that, but I have manually set limit to 66fps as that is lowest where it will go during online races.
Oh that depends from you monitor, if you have 1024x768 resolution and 19" monitor it is pretty safe to use 85Hz, 75Hz at least, you can choose this in LFS screen settings.
75Hz is considered at minium to not get headache and other symptoms, it is that flickering that causes problems, also some say it may launch epilepsy if you have something else already causing probability to epilepsy.
85Hz is considered what is that you should try to get and more is always better, but some monitors can't go so high.