YOU SAID:

I genuinely don't know who this guy is (he looks vaguely familiar?)

INTO JAPANESE

この男が誰なのか全く分からない（なんとなく見覚えがあるような？）

BACK INTO ENGLISH

I have no idea who this guy is (though he looks vaguely familiar?)

INTO JAPANESE

この男が誰なのか全く分からない（でもなんとなく見覚えがある？）

BACK INTO ENGLISH

I have no idea who this guy is (but he looks vaguely familiar?)

INTO JAPANESE

この男が誰なのか全く分かりません（でもなんとなく見覚えがあるような？）

BACK INTO ENGLISH

I have no idea who this guy is (but he looks vaguely familiar?)