Obama Explains Why Men Need Gay and Nonbinary Friends (From Personal Experience)

It's something like the emotional atmosphere or something's aura.

So a "vibe check" is evaluating how something makes you feel. If it makes you feel good, it passes.

It's like in the Beach Boys song "Good Vibrations".

Vibe checking someone is essentially looking how the person is (based on actions, looks) and judging them. The meme „didn’t know you were chill like that“ is the conclusion of a vibe check. Eg the person judges them to be a cool person in that case.

This is Instagram.

I know, I wish I didn’t know, too.