second screen resolution issue when coected to "Optimus"-configured laptop (onboard Intel and an Nvidia)

I've recently upgraded my Lenovo Y50-70 laptop from Windows 8 to Windows 10 and a very strange problem have occurred:

When I'm coecting my second display (VGA -> HDMI) — the resolution gets absolutely messed up.

I mean, it doesn't matter if I use the second display as the only display / extension for the main display / clone of the main display — I just can't get it right.

I set the resolution to 1680x1050 but the screen looks shrinks and there are two black borders on the left and right side, no matter what configuration I choose on the Display Settings window.

Some technical info:

1. The 2nd screen is a Samsung SyncMaster 2233 (model 2233bw)
2. My laptop's "system" specs: i7-4710HQ@2.50GHz CPU, 8gb RAM
3. My laptop's graphic card specs: onboard Intel HD Graphics 4600, NVIDIA GeForce GTX 860M