He vented his unhappiness ON his friends. He likes to vent his unhappiness on his friends > these are all good > meaning he "released" his unhappiness on his friends by showing anger to them , for example .

It is not unnatural but you need to use "vents" with first person singular. I googled it and found many examples. https://www.google.com/search?q=he+vents+his+unhappiness&oq=he+vents+his+unhappiness&aqs=chrome..69i57.20474j0j4&client=tablet-android-lenovo&sourceid=chrome-mobile&ie=UTF-8

"He ventS..." I believe the word you should be using is "fretted". "He fretted out his unhappiness..."

No. We normally vent our anger, resentment, rage, frustration or spleen on someone.