Is there a setting in LFS to adjust the level of cockpit vibration (the visual shaking)?

Yes, in Options->View: All "1g..." sliders.

Welcome to LFS, btw.

And maybe someone know the optimal settings. I'm searching it for a while.

Optimum settings will change with opinion....

Mine:
1g head tilt: 0.10
1g lateral shift: 0.030
1g forward shift: 0.065
1g vertical shift: 0.015

Look function: steer
Look sensitivity: 1.10