I tried really hard to disable IGP, but it led to Nvidia driver (!) refused to load. Weird.
What I did: put i915 (Intel driver) into /etc/modprobe.d/blacklist, rebooted, checked that Intel IGP is not "detected" by the system. So far so good. Funny moment is that now I cannot enable my Nvidia card. nvidia-settings doesn't recognize it, reinstalling Nvidia drivers didn't help.
Then I removed my i915 entriy from the blacklist, installed i915 driver, removed nvidia one, rebooted and voilà, - I have stable 59-60 FPS running any SFML apps with/or without vsync enabled in the code. What matters is that there's no more tearing.
Looks like having hybrid graphics has its drawbacks, or maybe it is an Nvidia fault. I still don't get why I couldn't force Nvidia to work with vsync.
